some sponges can encrust really fast and overgrow slower growing stuff such as corals. its not malicious but I would just be aware of it when your placing corals, keep it clear of your overflow vents. without knowing the species I cant tell you its exact habits. if its too thick on a rock you can remove the rock and scrub it off with a clean toothbrush or peel it off depending on the type. do this out side of your aquarium as any piece that breaks loose has the capability of growing into another sponge or recconnecting with another of its own type. you can also "air kill" sponges off rock just pull your piece of LR out that the sponge is on and let it sit in the air for a few minutes. "Air killing" will kill almost the entire sponge with the exception of the gemmules if that species creates gemmules.
Myself I love sponges and pretty much let them grow rampant. where ever they want.
