The mold is on the cell surface, but it's below the playing area. There do not seem to be any cracks in the finish-- I would expect the mold to collect along the cracks, if there were. But you can see why I want to stop it in its tracks before it creeps up to the playing area. It's not much, and I know it's mold only because my husband has a degree in biology, and he looked it and said "Yup, sure is." He offered to culture it in a petri dish to be sure, but I said it wasn't necessary.