I don't think you can keep them looking like they do in the ocean ... at least not without regular intervention such as scrubbing with a toothbrush, etc.
For algae, one option is to put all the snails together in a basket that they can't get out of ... and keep it in your tank/sump for a couple of weeks. Without other, easier food to graze on, they'll eat the algae off each other's shells.
The ocean is pretty different from out tanks...
Less nutrients means much less algae growth on shells. Stronger currents constantly abrading the shells like mild sandpaper. In deep water or in reef crevices, much less light fueling the growth of coralline on their shells. Etc, etc.
If the LFS didn't have constant turn-around in their stock of snails, their shells would look the same as yours.