It may be only my opinion, but it's not a question of if, but rather when. Crabs are opportunistic feeders that will eat whatever they can find. I have had two emeralds. The first attacked three corals within an hour of introducing. The second was well behaved for over two months before it ate a coral to death. Browse the forums and you'll find many similar stories. Avoid crabs. Which sucks because they're really cool.