Organism,
I completely defer to Bang's opinion.
I would say that in my limited experience, I had one that looked almost EXACTLY like that in my tank, and it was "living" in my zoas (OK, it was eating my zoas, and the zoas were not happy about it!).
Personally, unless you know for sure that they are ok (and that depends on what corals you keep), I would get it out of there.
Stewart