Black Sponge or Tunicate?

[Petrichor]

Got this as a hitchhiker on some live rock about a week or so ago. It was smooth, slimy, and uniform in color and texture when I put it in my tank. It hasn't moved at all, so I assumed it was some sort of sponge and would die off during my cycle.

Three days ago it started growing what looks to be little siphons (?), mostly on the top half of the mass, and that got me thinking maybe it's a tunicate and not a sponge. As of today there's been no more notable growth, however, I was playing with my pump angles and it might be getting more flow now so I'm not sure if that contributed to the change.

Thoughts? Anything to watch out for, or tips on care if it's a keeper?

 

[Brad Miller]

Looks like a sponge to me....Most sponges that are good, grow slowly.
Keep an eye on it, if it stays an irregular shape like that, and grows fast, it’s probably a bad one.

 

[Petrichor]

How fast is fast for a sponge? If it does grow fast can it be confined on the sandbed? The rock it's on is a a small broken branch, so it could easily be moved.