molds will normally only grow on porous surfaces that stay moist, if given the proper conditions. it WILL grow on wood in a new setup, but it is a white fungus that goes away on it's own in several weeks. polyurethane may have air pockets in it which show after it is carved, but it is still water proof. if that person is growing mold in their tank other than the cute, white, fluffy stuff, then they have other issues in the enclosure that should be dealt with. submerged "mold" would be a form of algae. some can't be helped, some can.