IME, when Emeralds are small they are relatively reef safe, but when they get some size on them they are your average crabs, opportunistic feeders that don't discriminate in what they take. I think larger ones will eat fish or shrimp if they can kill them. I have also seen Emeralds cut the tips of of Euphyllias. Haven't seen, but have read reports of them damaging Zoas as well.
Many folks buy them to eat Valonia (bubble algae). This is hit or miss at best, IME. I ended up pulling mine from my tanks when they got big. Too risky, IMO.