Brown growth ID

[Mebbid]

I have what I am fairly certain is a sponge that started growing on the bottom of my GSP and then transferred onto the rock the GSP was sitting next to. It's starting to get bigger and spreading so I am wondering what species it is if anyone can point me in the right direction.

 

[Sniperhank]

Picture one looks like a sponge... but I've spent my day cleaning my tank which is full of a fast growing coraline algae that is brown that looks like the second picture. If it starts "plating" then it is coraline.

 

[Mebbid]

It's the same stuff in both pictures. If you look closely you can see some tubes sticking up from it on the second picture.