Pyramidellae are the bane of a clam keeper's existence. There are wrasses that will eat them, but unfortunately, the snails come out at night when the fish are sleeping, so they aren't 100% effective. At this point, manual removal is all you can do. Remove the clam to a bowl or bucket of tankwater and go over it with a magnifying glass to remove any snails. Check beneath the mantle and the shell margins, and especially the byssal area ("hinges"). You'll also want to scrub any egg masses from the clam with a brush.
One problem is that these snails often live in the substrate, so you'll need to make this a nightly activity until you eradicate them.
HTH