You see, the funny thing is...I don't have any clams. My other snails (Astrea, Nassarius, Margarita) are not affected. I checked every one of them.
I posted up an ID on Reef Central, and they all seem to think they are Rissoid snails, and in fact not Pyramids. I'll keep a close eye on my other snails for parasitic action. I've never heard of Rissoid snails... here's a pic of the snails in my tank:
