Need ID

[DACJ]

I picked this hammer frag couple weeks ago, this white bony thing was on it and the sales said not to worry, it's not coral's damage, it was already on there before. After a couple days I realize there is something alive in it. It looked like feather duster initially, but then I realized it isnt...it is not round, more like a fan, and only comes out when I feed, unlike feather duster almost out 24/7, and the way it "hunts" for food looks quite aggressive, getting a bit worried since it's a meat eater. Just watch how it comes out to snatch stuff, acutally managed to catch a full size PE mysis, (big) and pulled it in entirely with a couple of moves then stops coming out for like 5 min. Too bad I had stopped recording by then.

Any ideas what this is? good or bad?

Video Here Please ID