Make sure you are looking at the hermit crab and not the recently molted off skin of exoskeleton of the hermit. Do you see his shell anywhere in the tank with him not in it?
I bet you he is hiding somewhere in the tank, still in his shell after he just finished molting. Its amazing to see, but they shed there skin like snakes and I have been fooled into thinking a fish tore him apart in the past. But soon found out that it was just an empty batch of skin that he just molted off.