We have this black stuff that has seemed to have invaded or tank. I thought it was black beard algae so I started taking the rocks out of the tank so I could kill the algae with extreme temperatures. Once I pulled out the rocks I noticed that it is not fluffy (best way I can seem to describe it) like other algae. I can't even feel it on the rock. It's like it's part of the rock. Does anyone know what this is so I can try and get rid of it.