Emerald Crab attacking my Duncan Coral

This morning, I had to relocate one of my emerald crabs after I watched him climb on top of my Duncan and start picking aggressively at the Duncan's fleshy bits. The Duncan has been closed up for a few days, and I've been trying to identify the problem. It has been really happy since I first put it in the tank a couple of months ago. I added the emeralds two or three weeks ago, and last week the Duncan started to act differently. I assumed it was reacting to my clowns and tang brushing it too often as they swam, until this morning. After I moved it, my wife told me that she saw the emerald pinching one of the clown fish a couple of days ago. So the crab is now banished to the sump.

I know that emeralds are supposed to be reef safe, but I read this morning that there are the occasional rogue crabs that do this kind of thing. I don't think it is an underfeeding problem. I feed the tank daily with a variety of frozen and dried foods, and I usually think that I overfeed the tank. I think I just got a jerk of a crab. I bought another emerald when I got this one. It is much bigger, but it hasn't shown any signs of this kind of aggression. After this morning, though, I'll be watching for it.

Has anyone else had this kind of experience with emeralds?