No way to tell without a picture. Long conical shells are common for ceriths.
Don't go crazy adding a CUC. Most recommendations on the net are extremely excessive and actually add to the bioload rather than help maintain it. The better approach is to have a few different species that are of different sizes to get to areas others cannot. In your size tank I would add 2 Mexican turbos, 5 nassarius (only if you have a sandbed), 5 trochus, an urchin, a brittle star and a diamond watchman goby. With that CUC your rocks and sandbed will stay clean. I would also consider dosing your tank with a mixture of different pods if you haven't noticed a significant amount of them. Stay away from Asterina snails. They will eventually die due to the fact they can't right themselves when they fall. Hermit crabs are debated on here often due to the fact they kill snails for their shells. I enjoy having hermit crabs, however, my approach is to keep just a few and of species that are unique. Like Halloween hermits and Hawaiian zebra hermits. Add a pile of different size empty shells and this will lessen the likely hood of a hermit killing a snail for its shell. White and blue hermits are the most lethal (in that order), while scarlet hermits are a little more docile. If you have not seen any micro brittle star and worms in your tank I would reach out to your LFS or fellow reefers and see if they will give you some. As far as true crabs go, the only thing I would put in my tank are porcelain and pom pom crabs. IME, emerald crabs get too large and will eventually supplement their diet with corals assuming you keep corals. Finally, shrimp. A bonded pair of cleaners are fun to watch especially when they spawn. Fire shrimp are cool too. Both will aid in the cleaning of your tank and fish. However, some species of fish will eat shrimp so do your homework.
Remember nothing lives forever, so a clean up crew will often need to be replenished.
Hope this helps.