Looked on Hitchhikers guide but no luck

[Picesduh]

Today I found this quarter sized black substance between two pieces of live rock. Any idea what it could be?
You can see holes in it but it does seem to be a living thing but not sure what.

 

[swebb]

Looks like a black tunicate (Phallusia nigra)

 

[P-Dub]

That sir is most definitely a sponge. https://www.sciencedaily.com/releases/2010/08/100804151404.htm
As well, I see them while diving all the time.