I'll be honest... same, never had an emerald crab go rogue. Last two were from Reef Cleaners and never harmed a coral.
I had two emerald crabs in a 20 gallon cube. One was biting the polyps on my torch so I removed him. I actually saw him lights out grab a polyp with his claw.
The other one I thought was fine until two months later I saw the other one do the same thing. I think the problem is I only had one head of torch so it only had one target. It did have algae for them to pick on too. I haven’t been lucky with them so if I do add more it will be pithos crabs

