How do I get rid of mold on roughcast?
Vinegar is the best option to remove the mold. The vinegar will penetrate the porous surface and remove the underlying membrane as well as the mold growth on the surface. Spray the vinegar on the surface and let it sit for an hour, then spray again and wipe off. Bleach is only good for removing mold on the surface, it doesn't penetrate the surface to kill the roots.