Laura,
First off if you are attempting to sand mold , I hope you are wearing respiratory protection. That being said , sanding is not the wisest way to mitigate mold, even dry mold. A simple solution of bleach and water should take care of it. Use a scrub brush or even a spray bottle, rubber gloves and some sort of facial protection if you are working overhead. I am not sure of the ratio, but I am quite sure you can find it. Don't sand it , you will just spread it around.

Why worry about the mold on the wood surface? There is usually more mold inside of soft or porous materials than outside. Sanding is unnecessary and probably not advisable because of all the dust. Just wipe it down with a 10% bleach solution (0.6% hypochlorite). Check into boric acid (borate). It should work to suppress mold and is fire-retardant. It migrates into the wood.

Stay away from flavored bleaches. The hypochlorite concentration is lower so as not to destroy the flavoring chemicals. A few years ago all bleach was 6% hypochlorite, but now, you just don�t know. If you forget the 6% part when at the store, just pick up a jug of germicidal bleach and look at the concentration. Then go for any brand that is marked with that concentration or higher. If you don�t see the concentration, go for another brand.

Bleach is not a particularly good disinfectant for porous surfaces, or material that is oxidized by the bleach, so wood is not a particularly good target. It will work to some extent, but not very well if the mold is extensive and penetrated the wood deeply.

Random orbit sander. They can be found in the box stores. This will probably come in handy for other projects. If you really want to get aggressive, belt sander or a 4.5 "
right angle grinder with a sanding disk attachment. I don't think you would want to hold a belt sander overhead.
I tend to agree with ionized's summation, besides its a lot less work.

As many have stated... mold weather on drywall, deck, fence or anything else can easily be mitigated by mixing bleach and water in a pump sprayer and spraying it on the affected area. If you would like more info on this simply visit my site and shoot me an email on this subject or any other home repairs for that matter. www.marblefallshandyman.com

Here is a link that might be useful: JCK Home Services Handyman Serving Austin Texas and the Hill Country

Bleach will only kill the surface mold on porous materials like wood, but not the roots. The water will penetrate into the wood and actually feed the mold roots.

Counterproductve, eh?

So while bleach can work on non-porous materials, you need a different method for porous materials.

I don't recommend sanding. It seems that all that is being done is the spores are being sent airborne to travel everywhere. If you're sanding to get rid of the stains, that's for pretty-factor. But I recommend killing the mold prior to sending spores everywhere.

Bleach isn't recognized as a mold killer by the EPA, and even Clorox only recommends bleach for use on non-porous materials like tile.

If you still want to use bleach, look at pool bleach. Generally it's 12% versus the 5% to 6% that you find for home use.

Spraying bleach on a floor joist to kill mold or fungus is literally the dumbest thing you can do. Fungus and mold grow into the wood. People have been doing it for years thinking they were solving a problem and saving money but it is actually the exact opposite.

It might temporarily kill what’s on the surface, but it will come back stronger than before. If people would read the instruction it says “for non porous surfaces.”

Fungus and mold are a symptom of moisture. When you spray bleach water on joist your really just adding more moisture to it. Borate (boric acid) works probably the best.