Vermetid snails are marine snails that don’t particularly look like snails.
Most people who have them in their reef tank might not even know that the Vermetid snails are there.
Vermetid snails tend look something like pieces of tubing with some long threads hanging out of them. Those long threads are snail mucus that the vermetid snails use to collect food and pull it closer — these snails are really, really lazy animals, and don’t move around even to find something to eat.
Even if people notice them they do not suspect that it is a snail. But just look closer and here it is.
Here is another great video (this one is unfortunately not mine) on Vermetid snails on YouTube.
I usually keep mostly soft corals and some LPS so I don’t care very deeply about Vermetid snails in my reef, but there is some evidence that they are harmful to some stony corals: