IMO the wrasse is not nipping at the clam intentionally you may not be supplying enough food for it and when you feed some food is landing on your clam, the wrasse can then irritate the mantel feeding off of it and and this stresses the clam
How long has the clam been in the tank? It looks like it has decent growth on the shell, but I can't tell how close the far side is to that rock or if the rock is touching the shell based on that picture. A clearer picture might help, but from what I can make out of the mantle along the scutes, it looks like it might be "pinching" it up a little bit - I really can't tell from the picture though. Just the blue area (black), look like they have small folds in them along each scute.