It can happen
It's bad to let a painting hang around in a moist area, even if it varnished. Mold grows on unvarnished oil paintings, it seems to be attracted to the dark Sienna's, Umber's and Ultramarine Blue.
If you live in a moist area, paint with acrylics. Mold does not effect dry acrylics 100th as much as oil paint.
Mold can be removed with alcohol, bleach will also work. But it won't replace paint! Mold will not form in wet acrylic pigment when ammonia is added.
Ammonia gas will evaporate and leave only water.