You posted a few things that are very disturbing. Mold problems are a serious issue. My advice is to first determine the root cause before trying to apply a fix.

Did you have the mold problem before the foam was installed in the attic? You said the foam was sprayed over fiberglass insulation? Did the foam company remove the old foam and insulation? Do you have confidence they have done the job correctly?

I can understand advice about adding new returns, not but about blocking old returns. I feel the HVAC company is guessing at a solution.

Do you have mold on the other side of the sheet rock of the stairway ceiling? If you do then the source of the problem is in the attic. Adding or subtracting returns and vents in the house is not a cure in my opinion.

Claiming no expertise on fungus, my observation is that visible fungus growth does not occur at relative humidity levels of 50% or lower. And visible fungus growth is slow at 60%, virtually certain to be quite evident at 70%.
Interestingly, rust (of iron) forms very slowly at levels of 50% or less.

We are in New Orleans where it is humid all year and mostly hot. We purchased the house last October and there was no mold from then until 2 months ago, when the severe heat started. The foam company removed every bit of the old fiberglass insulation and we are confident that their job is correct. Above the stairwell ceiling is a 6 inch or so layer of foam and then roof, no attic. 2 attics border the walls, but not the ceiling. Our layman's take on the problem is that the cold air from upstairs and downstairs meets the hot air in the stairwell and forms condensation, which in turn leads to mold. Adding a vent to introduce cold air seems like the common sense fix to us, but again, we aren't a/c people. All we know is that we've spent almost $2000.00 on foam and an a/c return and the problem is worse. Thanks again for any and all advice!

When the specks of mold form, they can be easily wiped off with no remaining residue. This leads us to believe that the problem is surface and not coming through the sheetrock.

stairwell

Newly installed return that made things worse. By blocking it with cardboard and running the oscillating fan, the mold problem is solved. But we don't want to run the fan 24-7!

Upstairs space consists of a 12x14 bedroom, 7x10 hall and 12x7 bathroom. Stairwell leads to the bedroom. Hall and bathroom are comfortable, but bedroom more difficult to cool. We think this is because of the heat from the stairwell going straight into the bedroom. Another reason why we think an a/c vent in the stairwell is a good idea...

Plain and simple, you must find the source of the humidity.

Then take the appropriate remedial action.

You have told a nice anecdotal pictorial story but few facts.

What is the inside humidity?

IMO

I have many facts....

1) Stairwell with skylight and no ventilation

2) Mold problem occurred only when heat of summer came

3) A/C return installed my a/c company to fix problem made it worse

4) Mold is surface only...not coming from roof or attic

5) House is located in extremely humid climate

I do not have a humidity reader. The a/c people had one, and said the levels were not extreme, but then, what do they know?

I am on this blog for help and advice. If I knew how to fix my problem, I wouldn't be soliciting the opinions of others. Any and all advice is welcomed and appreciated.

Thanks!

"a/c people had one, and said the levels were not extreme"

Go out and buy a hygrometer. From this statement I would infer you seem to have a humidity problem. You want the humidity level to be below 50%.

Are you getting condensation on the glass of the skylight? You stairway is very common set up with the exception of the skylight. How cold does it get during the night?

I can't see how adding a return would make the situation worse. Does your AC run in short bursts? What temperature do you set the thermostat?

Blower setting should be checked and dropped down to next speed.

Fan setting at thermostat should be on auto not fan on.

IMO

"I can't see how adding a return would make the situation worse."

Maybe added pressure differential that sucks in humid, outside air.

Thank you all for the responses!

Fan is always set to auto.

Skylight has never had condensation on glass.

A/c is set to 68 at night to get upstairs bedroom to 72-73. Any lower and it won't cycle.

Note: There is no return air in upstairs bedroom. Could this be part of the problem?? This is the one and only room upstairs
with a bath and vanity. Thanks again

the foam is on the roofline, and has been redone
due to being sprayed on fiberglass batts...right?

were the fg batts on attic floor...I know we don't put
batts on roofline.

IF the batts were on the floor..when roofline was
re-sprayed...did they move the batts?
was the roof to attic floor seal made??..seems I asked
this in your other thread.

pics of eaves/soffit where roof meets attic floor
from inside the attic would help. (again)

as for the skylights..was the foam installed around them
at the roofline & did they continue the foam on the
back side of the sheetrock?

a simple blower door test would show how much
air leakage wasn't sealed in the attic.
testing first with attic hatch closed to determine
a baseline, and then with hatch opened.
the difference would be attic leakage.

the blower door test depressurizes the house
to show how much air leakage the house has.
it is a good idea to air seal the house..you won't get
it too tight. it will also cut down on those Entergy bills,
if you do the air sealing.

call entergy, there is a giveaway program for n.o.
get your free audit...and while they are testing
open the attic hatch & find out how much the attic
leaks.
don't expect much from the person doing the test...
they get paid by your utility provider.

I test foam jobs often. instead of just measuring
the leakage, I mark the leakage spots with spray
paint for foam co to come back & actually make
the seal they claim to achieve.

once you've addressed the moisture issues,
then getting RH to 50% will be the next goal.
you may be able to with a stand alone dehumidifier...

what size is the hvac system..a/c with gas furnace?
and what size is the house?

you have bigger issues than return air at this point.

answers are needed.

best of luck.

The return air vent was installed in the wrong location (Too low.). Heat rises up the stairwell carrying humidity. Had a similar problem. Our upstairs is on it's own HVAC system so installed a return on the back wall at the top of the stairs high as possible near the ceiling and a supply in the ceiling at the other end of the stairwell. Also had returns installed in ceilings of upstairs bedrooms and back wall high as possible in the bedroom closets. With only one zone of heating and air, you should upgrade to zone the upstairs separate from the downstairs with it's own thermostat wired in parallel to a humidistat. Thermostat in a bedroom and thermostat in the upstairs hallway. Live in Georgia so our summers are hot and humid. Needed more conditioned air movement to remove air trapped at the top of the stairwell and better circulation in the bedrooms and closets.