What is this stuff?

[Coasting]

Its really annoying my GSP, is it some type of sponge or algae?
It makes it hard for the polyps to come through it. Can I just pick it off or is there a better way to get rid of it. It is only on this Piece of gsp thats in my sand, so I'm assuming it came in with it, its slowly grown over the past 3-4 months to this size. Now its just really bugging me because I want the gsp to spread, not this stuff

Thanks.