I have no idea what these are. At first I thought they were new growths of my corals. And then I found one attached to the rocks. So now I’m lost. What is this???
Vermetid snails, hitchhikers that are unwanted. They usually increase in numbers due to elevated particulates/nutrients in the water column. The web they cast out can be irritating to corals. You can seal the ends of them with cyanoacrylate glue which will starve them out.