That is a good question! I'd have said aiptasia, but you're right, it looks like they have branching tentacles. My best guesses are a sponge or maybe xenia, but it doesn't really look like those either.
You not looked at it after lights out with video on phone with light on phone ? As polyps be closed so much easier to see or even get a turkey baster and blast water on them to close the polyps when tank lights are on