My first thought was the same as Nova's, pyramidal snails. However, you've had these clams for awhile, so I really don't think that's it at all, especially since the wrasse is nipping at the mantle of the clam. What's odd is that the wrasse isn't nipping at all of your clams. I'd keep an eye on the clams. He may just be eating pods off the clams shell, etc. It doesn't seem like the clams mantle is damaged much by this little nipping he's doing, but it's worth monitoring.
They can be known to aggravate ornamental shrimp and small crustaceans, so it's not out of the realm of possibility, that the wrasse is simply nipping at these clams because he can. It's one of the issues with these wrasses, they're considered "reef safe, with caution," which basically means, put them in your tank at your own risk. No one really knows what they might do. Some behave, some destroy, some are aggressive towards other fish, so much so that some have been seen pecking the eyes out of other fish, while others are model citizens of the tank. Unfortunately, there's really no way to know until it's too late. Heck, I had a clownfish that ate coral on me, so anything is possible.
He's not going to be easy to catch, and your best bet really is to find where he's burrowing in the sand at night. When I bought my leopard wrasse off of a fellow reef hobbyist, his method for catching him seemed to work:
They're also sand burrowers at night. He turned all the lights off for a few hours before I came to pick up the wrasse. When I go there, he fed the tank, and popped the lights on. The wrasse immediately came out for food because he hadn't fed the tank in 3 days, prepping for this. As soon as the wrasse came out, he scooped him out with a net.