I have some fake hollowed out rocks in my aquarium for about a year now and I never moved it. It sits firmly into my sand so no light can get in. Today I was cleaning and lifted it up to rearrange and all this black mold like fuzz came out and a lot of it floated into the water. A lot of my cichlids swam by and ate a chunk full of it as I tried to siphon it all out. What is this and is it harmful to the fish??! I thought algae only grew with sunlight but there's no sunlight that would have hit inside the hollowed out rocks. Help!