Mold can not and does not grow underwater. What you see is most likely fungus or algae.
Thank you for that. I was reading the same thing and shaking my head. Mold will not grow under water it needs moisture yes, but not submerged. It's fungus. My manzanita driftwood started growing all sorts of fungus and algae recently and it was used in an aquarium for 3 years and left out for a year. I threw 20 ghost shrimp in there and they are taking care of the problem for me.