I closed the drain and made a 50% mixture of bleach and water. It slowly leaked through the "plug". I wiped the residual mold around the drain plug away.

Tonight it is back.

Any suggestions on how to treat?

Do we need to put a new pea trap and pipes above?

The sewer pipe that served this bathroom was broken this spring and since was repaired. Is it related?

It gives me the heebie jeebies...like something from an Alien movie! Please help!

Common trap or separate?

If a common trap is used check and see it the drain line is partially clogged beyond the trap.
One sink will fill the drain line for the other, and then the water will slowly drain away past the partial obstruction.

Bleach can only kill things that are present, if the source of the mold remains and the line is not cleaned it can quickly return.

You may need a large 'bottle' brush to scrub everything clean.

Unless you remove the trap and close off the line from the sink any bleach solution is going to run off before being very effective.

Boiling water will have the same issue.

Neither will contact the entire surface inside the pipe.

I've relayed an incorrect message. There are two separate sinks, 30" apart, with separate drain pipes and pea traps that join in the wall out to the sewer line (which was crushed this spring and then replaced). We had lots of back up in this bathroom's tub and separate shower.

There is an overflow drain in each sink.

Since last posting, we leaned on the hope that the mold was caused by the back ups we had this spring. We tried vinegar and baking soda (foam).

Should I force something to clean the overflow drain?