That isn't an emerald crab. It reminds me of a 'strawberry crab' I saw a picture of a while back. The hairy legs on crabs in general usually means they aren't very reef safe...the jury is usually out on emeralds, as they also have hairy legs, and can be fine or they can go postal on corals...
That being said, the crab doesn't look that equipped to take down a fish. Under normal advice, I always say that if a crab is caught munching on a fish then you caught it in the reason you put them in your tank...to clean up!
The only bit of concern I have with this guy, is the size. The size of a tennis ball? That is outrageously large. I'd move him off into a sump, if available, until more information can be figured out about this guy.