A tunicate, huh? Well it's certainly a possibility.... it doesn't really look like any photos I can find except maybe like a "clubbed tunicate". I'm still not finding much info on them, though..... I'm not even sure if this thing is dead or alive, or if it's good or bad But since it's in a spot in the tank where the fish seem to like to congregate it can't be too anti-social so - it stays!
If anyone else has other guesses as to what it might be (from this planet or otherwise), keep them coming. Until then - I dub him "tunicate".
Sue