I have a couple of sponges they just sit around and grow a little - mine are pink and brown, respectively.
I found this in an article, though..
"The sponges here don't look like Sponge Bob Square Pants. They also don't look like the square sponges......."
WAIT.... that's not it... here we go....
"Some sponges grow on the undersides of corals and help hold pieces of corals together like glue. Other sponges can damage the coral. These sponges have an acid that eats into the corals. These sponges bore into the corals and can cause them to break apart. "
Here's the link.
So now we gotta figure out which sponges are the acid-eating kind... and mine looked so harmless! 8O
Hope this helps a little.