It just started growing recently and it grows pretty fast. I still would like to know what it is but I took the rocks out that it was on and set them in the sun.
I love funky stuff like that. Can you get a close picture?
I'm betting sponge but could easily be an algae.
A simple test is when you rub it between you fingers sponge is a little slimy and kinda crumbles algaes is like plant if it's totally squish and gross totally rare bacterial colony.