Vermetid snails. When I see them, I crush them with a small pair of coral sheers. They are really common and usually aren't much of an issue. However, I have seen some infestations so I crush them when I can get to them. Come folks will put a dab of super glue over the hole. I've never tried it but it certainly sounds like that would work.
Edit: I just want to reiterate that I kill them when I see them but still have plenty in all of my systems. These are not something that I would spend a lot of time trying to find and eliminate. Their population will naturally ebb and flow and the only tanks I've seen with "plague level" infestations have been the result of heavy coral feeding. These little buggers extend a slimy web that catches food from the water column. If you do a water change and notice a gross little streamer coming from a rock, chances are it'll lead back to one of these dudes.
