I am wondering how big they are, I am doing my wake up at 4 am and look at the tank thing right now.. lol.. Turns out I have a bunch of little snails as well, They look identical to yours, but some are green shelled some are white, they arent in the corals, but on the glass and rocks. very tough to get a picture as they are literally about this small "O"
You can see in the above photo how small it really is, maybe1-2 mm at best, I have been looking online and since mine are seeming to be nocturnal, I am steering towards Collonista amakusaensis which are harmless algae grazers, its tough to say what yours are exactly as well as there isnt a size reference. From what I am gathering here mine at least seem to be harmless, I would like to find out what size yours are roughly. If I am correct in this ID they are a member of the tubinidae (turbo) Classified as mini or micro turbos.
Very tricky to focus on these guys, but upon flipping them to look at their mouth area, they have the same "calcareous operculum" or what I like to call their front door, turbos have these too, it looks like a helmet kind of and when they retreat inwards this becomes apparent. I would try to get one of yours to see, if they have this "helmet" odds are its a turbo species and you are in fact safe. Any one agree or disagree?
http://www.aquaticcommunity.com/SwSnails/Collonista.php
Here is where I ID'd them, I thought perhaps sundials or heliacua at first, but as neither shows signs of a flat shell, and is showing the characteristics from what I am gathering online here, I doubt that is the case. and the fact that mine have the door/helmet kind of ruled it out, if you could find a characteristic on yours showing this, and describe it, then odds are in your favor I believe. Man.. that was some light typing for going onto 5am... lol