Diddo on what Martin said.
That stuff is VERY slippery though. I had a bunch of that crap all over a piece of live rock a while back. It was very slick and velvety looking, like a deep chocholate brown. It also grew in like a plating form for the most part. I couldnt pluck it off with tweezers though. I had to scrape it off.
One day it just all started died off, I have no idea how i got rid of it either! LOL!
I was told it was a form of coraline but I dont think it is.