What is this (algea)?

[aras]

I have this thing on one of my rocks. I think it's been in some dark areas on a rock for a while, but recently I noticed that it's been spreading...
It feels soft and slippery, but at the same time pretty thick and hard to remove (requires quite a bit of toothbrush action).

 

[saltyfilmfolks]

Its a sponge.
A pretty one too. Very good clean up crew.

 

[aras]

Thanks. I would have never guessed... I was actually making plans how to nuke it! Will leave it alone now