Yes, it's a bulla stage cowry, and is harmless. If it survives, it will continue to expand the shell untill the characteristic teeth form, and the dorsum thickens. It looks like it might be a juvenile Cypraea onyx or C. walkeri, but ID will be a lot easier as it matures. Cowries spend a lot of time hiding when they are juveniles, and are much more vulnerable to predators like crabs and fish when they are thin-shelled juvies. Good luck!
Don