While I've read that red coris wrasses are beneficial to clams, in that they'll keep them free of pyramidellid snails, my experience was otherwise. My juvenile red coris flipped over a new maxima and ate the guts from underneath along with my melanurus wrasse. I tried adding a crocea later and he started pecking at the byssal area again, clearly irritating it. I removed it to my other tank until I can catch the wrasse. The melanurus has already been sold.
I do have a large derasa that he doesn't bother and a crocea that is tightly bonded to a rock and the wrasse can't get at the byssal area so that one is safe. So, if you can get the clam bonded tightly to a rock, red coris wrasses are probably safe, but if it's loose on the sand, I wouldn't try it.