Some kind of algae, probably. I'd Scrub it off and apply some peroxide carefully to the shell with a q-tip to keep it from getting into my tank. Just don't get the peroxide on the snail soft parts... not sure how it would react.
I had the exact algae growing on my trochus. Make sure you scrub it really well. I thought I did but a month later it grew out to a nice 1 inch piece of macro. I actually thought it was pretty neat looking. I would follow what Idoc said