can’t see the video but you sure it’s an emerald crab? And if so you sure the hammer is healthy? Any algae on the skeleton? Sometimes it may look like they’re eating the coral but just munching algae on the skeleton. I’d still sump the crab either way but I’ve learned with saltwater cuc will do their job and sometimes that looks like they’re finishing off an already dying animal.
Emerald crabs feeds of algae and detritus, if they are eating your hammer coral. It can only be two reason, one is that your running low on food for them in your tank. When food supply is low they tend to go after snails or softies. Two is that your hammer coral is already dying. My pepper mint shrimp and nassarius snail did that to my torch and my maxima clam as well, it turn out that both was on its way out.