nah, they aren't like boxfish, no worries. also, the drop from the top of the tank would not have hurt him at all. water slows down the fall and no doubt they make that fall all the time in the wild. i'm not sure but can't emerald crabs turn into fish-eaters? i think so but someone else might know better. just something to think about.
Figuring out how to keep my fish healthy is like the pethagorean theorem; there is no solution.