Duncan coral missing half a head?

[Thepopo]

Yet another unsolved mystery in my tank. I woke up to find that my Duncan coral was missing half a head. All the little arms on that head have been eaten clean. I don't have any fish in the tank and the only thing besides corals is a anemone crab. What could have eaten him? Nudibranch? Any ideas are welcomed.

[DON CORALEON]

gorilla crab

[Sisterlimonpot]

Sounds like you have an unwanted hitchhiker, Get yourself a red flashlight and watch the tank in the dark hours and be on the look out for crabs or nudi or something that is making a meal out of your corals.