The Vermetid Snail is a very Unwanted Saltwater Aquarium Pest, A Coral killer, that is a stationary gastropod, often red, brown or purple in color, that has a hard shell that is cemented hard to your rocks, corals, glass, and other snail shells.
Vermetid Snails will slowly kill your corals by smothering them with the mucus webs they cast from the ends of their open tube shells, which are used to catch food and nutrients they need for survival, the webs often irritate surrounding corals, causing polyps to remain closed eventually ending in tissue loss.
Vermetids also hurt or slowly kill corals by stealing their much needed calcium, impeding their skeletal growth! Often the Vermetids will attach their selves to a coral causing lesions that will result in tissue loss and tissue damage, eventually killing the coral if not removed.
The best practice is to manually remove the Vermetid Snail, using a razor of sorts to cut the snail from the rock, coral, or whatever it may be attached to from the base of the snail. The snail is not in the tip of the tube, its actually down in the base of the tube as you can see in the picture.
Some aquarist will glue the tips of the snail closed, which will kill the snail, eventually starving it, but after doing this over a period of time, you will end up with unwanted nutrients in your system due to the dead creature still trapped in your tank.
There is much controversy about different types of Fish, Dips or Treatments to kill these snails, but from most of the research I have done, I have come to the conclusion that its just best to manually remove them.