Arg....server ate my post. Trying again...
These are Lysmata. They are also not L. californica, which was going to be my other guess in addition to what Aquascaper mentioned. I think they are actually the same as the ones I have in all my tamks, but haven't checked to make sure. At any rate, since they are Lysmata, if they are picking at corals then there are three possibilities really: the coral was fed and the shrimp wasn't, the shrimp has some kind of other dietary deficiency, or the coral is actually on the way out and you just can't see it yet (but it will smell of it to animals in the tank). The last case is quite common with a lot of Crustaceans.