I had one for a while before I banished him for chowing on xenia. He was in a tank with squamosas and I never saw him bother them specifically, but I didn't really give him the chance to. IMO emerald crabs are an iffy bet in reef tanks.
Do you want because you like them, or is this for a bubble algae problem?
I have 3 in my tank. I got them for algae control, but I don't have algae problems like I did before. They havn't bothered my xenias yet and I hope they won't. They'd be a pain in the rare to catch to. One of them ate my electric flame scallop once. With my lighting I'd have to place my clam way up top if I got one. I'm debating on it still. Thanks for you opinion/advice. Hoping to get some more out there.
I had added 2 of the emerald green mithrax crabs to our 125g because we were getting bad bubble algae and supposedly they eat it. Well, I never did see either of the crabs go near the bubble algae, other than crawl over it...but they did do quite a job on our nice pulsing Xenia colony, chopped them right down!
We would have removed the crabs for that fact alone but they both somehow vanished for unknown reasons, never to be seen again. They were in the tank for about 3-4 months. I'd say don't bother with them at all.
I haven't had a problem with them eating my xenias. Maybe I'm lucky. My LFS owner says he has a few in with his clams and they haven't bothered his. He says all critters and fish have there own personalities and its always a risk. $40 for a low light clam might be a expensive risk or should I say treat for them. I'll post what happens if I decide to get one.