Nice close-up. The snail appears to be a small Marginellid. They are carnivores but, as far as I'm aware, not parasitic. So, your clam is likely safe from this little critter.
Marginellids, especially the "peppermint snail," turn up from time to time in the reef tanks. If there is just one or two cruising around, their impact is likely negligible.
I say all this with some caution however. There are many species (large and small, tropical and temperate, all around the world) and, as a whole, the group is poorly understood.
Cheers,
Bob