They could be rissoids, eulimids, or pyramidellids (and probably a bunch of other -ids, but those are the three most likely). Rissoids are harmless, and don't need a host, so that's most likely what they are, if they are on your rocks. If they are rissoids, the shell should have some crosshatch sculpture on it. Eulimids are smooth-shelled, glossy, and are usually parasitic on echinoderms (most often holothurians, but also on starfish and sea urchins). The pyramidellids that typically show up in reef tanks are parasitic on other molluscs, and may closely resemble rissoids in shape and sculpture. However, if they are pyramidellids, they should be associated with your snails or clams rather than the rocks and sand. Hope this helps,
Cheers,
Don
