I had an emerald crab for one night. He was in my tank for less than 2 hours before I caught him eating my zoas. He finished off at least 3 polyps before I shooed him away, figuring it was an isolated incident and he just hadn't found the algae yet. An hour later, he was eating my torch. And I totally mean eating. Finished off several tentacles before I caught him and returned him to the LFS
While Emeralds will eat the stuff we don't want, crabs are opportunistic feeders. If they find yummy corals and aren't currently stuffed full, they will eat those yummy corals.
It's too bad. I think they're pretty cool looking. I won't risk another one, though.