Vermetid snails
Sessile snails that cement a hard tube into the rock and feed by casting a mucus net into the water. The tube itself is harmless; the net is not. Where it repeatedly sweeps a coral, that coral grows slower and dies back — measured in the field at up to 81% less skeletal growth and up to 52% lower survival.
Make sure that's what it is
A small hard tube opening flush with the rock, and — the actual confirmation — a fine mucus net drifting from it, most visible when the flow drops. People commonly mistake the tube for a worm or a piece of coralline. If it retracts when you pass a hand over it, it is alive.
What it looks like
No photos of Vermetid snails yet. If you have it in your tank, a clear shot here is what the next person will compare theirs against — it's the most useful thing you can add to this page.
Getting rid of it
In the order we'd try them. Every method cites where it comes from, and where the reason a treatment works isn't actually established, it says so.
Block the aperture, do not just crush the tube
By handWhy it works
The damage is done by the feeding net, not the shell. Vermetids are sessile and feed by extruding a mucus net that drifts and repeatedly contacts whatever is next to it, including coral tissue — field experiments removing vermetids from around corals measured up to 81% more skeletal growth and up to 52% better survival in their absence, averaging 56% and 40% across species. So the objective is to stop the net being cast, and sealing the tube opening does that whether or not the animal inside dies.
What to do
Cover the aperture with a dab of cyanoacrylate gel or a press of epoxy. For tubes you can reach on a moveable rock, crushing the tube back into the rock works as well, provided the opening ends up closed.
Source: The vermetid gastropod Dendropoma maximum reduces coral growth and survival (Biology Letters, 2010)