Please help id this

[ricky1414]

What is this brownish-maroon mat growing like a carpet on my LR? It is in front of the snail. Excuse the large bubble algae at the bottom of the picture, its now gone. I know it is not diatoms, and i know its not cyano. I cannot brush it off of the rock. it started of about 3 months ago small, half the size of a dime, and now its like 15x the size. Is it some sorta flat sponge or dark colored corraline?

 

[Anonymous]

It looks like a form of coraline to me. I had red in my tank for a while.