Vermetid snails. Major pests. Some are bigger, some smaller, but you will be able to track the net back to the snail. They are immobile and attached to your rock. Nothing that I'm aware of eats them naturally without some sort of assistance. Anything that eats worms and inverts will have some interest after you break the shell, but it is MUCH easier to just super glue the trap door shut. Get head of these things before they take off in the tank, they won't hurt fish but these nets can stress out corals to the point of tissue recession and even death.