It's a brown wafer algae and it indicates good water quality. Don't ask me how I know.:mad2:
It usually will peal off in sheets, but not always. I took the worst rock out of my tank and pealed/brushed it and it didn't come back. I still have some golfball size spots here and there and it's under control so I'm not too worried about it.
btw, it will suffocate whatever it grows over/around so keep it in check