You can try looking up "Convolutriloba," which is the genus of photosynthetic flatworm that people often find in their tank. I find it is best to just siphon them out. If it is the discarded foot of a Stomatella, the snail is likely safe in the tank and you can discard the foot. Stomatellas and the limpets are harmless herbivores, so they are good to have, but weren't likely to be in the live sand you received (but perhaps the hermits, snails, and coral.) Feel free to post some additional photos if you want to get a specific ID.