To the best of my knowledge this is a fireworm, or at least one variety. Came in on small piece of rock with a caulastrea furcata coral (aka kryptonite candy cane). The coral wasn't doing well in QT, lost a few heads but no evidence of brown jelly disease. I lifted the frag rock up and found these buggers. The coral recovered nicely once I remounted what was left on a couple frag discs.