With crabs, it tends to be hit or miss either way. Most crabs are what we call "opportunistic eaters". Meaning, if they don't have a food source resource large enough to support their diet, they will substitute whatever else is needed with other things. For instance, Emerald crabs eat different types of algae and whatever else is on your live rocks. Some will eat detritus and leftover foods from feeding time if its available as well. But, if that wasn't enough to suffice, the crab will start to go after Small Fish, Polyps, other Inverts, etc.
I'm not saying that Emeralds are bad in any way, I have them in my tank and they would be the only ones I would trust. Just be aware of the possibilities.