Need ID

[Flacricket]

I have some brown stuff on my power head and 1 rock. It grows in a circle and feels like leather it is dark brown and layers kinda like monti. cap but always round and stuck on good has to cut off with a razor. I will put a pic later if I can get a good one. Any idea?

Cricket

 

[Dude23455]

I've seen that stuff before. I believe I was told it was a type of Macroalgae. But that was a while back