I saw your other thread about this problem too. This could definitely be the handy work of a mantis or pistol shrimp. In my opinion/experience, this is not the work of an emerald/mythrax crab. Emerald crabs are by and large just fine, but occasionally one of them can develop a taste for some corals, Zoanthids, for example, seems common. I don't believe that emerald crabs typically take down fish.
The other thing to consider is that you found your fish half eaten, but that's not proof that the crab killed the fish. It's only proof that the crab was doing what crabs do, scavenging. Meaning, the fish may have been killed by something else or passed for whatever reason, and the crab just happened upon the carcass, which is what crabs do.
Lastly, are you certain this isn't some fish parasite claiming the lives of the fish, and then the scavengers are living in their little heaven?
Good luck!