Who ate my brain?

I have a 75 gallon fish/reef tank that's been up for 6 months. I added
corals for the first time about 1 month ago. Today, I came home and a
lobe of my Trachophyllia open brain coral on the sand had been - it
looks like - chewed off. The lobe was down to white skeleton in about a
1/2" or 1" round area from one edge in. Overall brain is about 5" by
3". It had been looking fine, so I don't think it was spontaneous
decomposition; pieces were still hanging off, sort of like if you tore
cloth or skin and edges were flapping. The rest of the brain is alive
and still eating.

So: the question is who did it, of the following [I suspect the crab]:
Green emerald Mythrax crab
Cleaner shrimp
Peppermint shrimp
Fire shrimp
Black sea cucumber
1/2" blue leg hermits
Various snails (turbo, nassarius, Margarita)
Purple porcelain crab (filter feeders, so I don't think it's them)
Lawnmower blennie
Firefish (neither fish ever shows interest in corals)

I think this happened because about the same time I added the corals, I
got some turbo snails who really cleaned up the algae in the tank. I
think maybe somebody got hungry. Would it even be possible for it to be
any of the other creatures besides the crab? I've pulled him out of
this tank and moved him to my quarantine tank, if it's likely him I'll
donate him to the LFS based on this non-reef-safe behavior.

Also, any chance the brain will regrow its "flesh" over that eaten area?

Thanks for your guesses!
[Sorry if this is a duplicate post, my newsreader seems to have eaten
the first try.]