I've watched ceriths and turbos lay eggs in my tank, and both types left long, coiled strings of tiny, tiny eggs. My guess is that those are a group of babies, either limpets or stomatella (hard to say).
I also remember reading about pyramid snails - white, and about the size & shape of rice grains, if memory serves - that parasitize and kill clams. Judging from the discoma next to them, these aren't even that big, but if you have one or more clams and can't make a positive ID that they're harmless, I'd suggest getting rid of them just to be safe.