I had this stuff in my tank. It's a form of algae. I don't know the name of it, but after i put my tang into the tank, that's the first thing he went for. Picked the rock clean of it. Not too sure what it is, but it is a macro. My LFS has a ton of it.
It's attached to the rock quite firmly, so you may want to use a razor to pull it off if it's undesireable, kinda like fragging zoas.
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