YEs, what I said above. They are gastropods that have their shell stuck to rock work. They can irritate coral. Just do a google search for vermetide snails and you will see them.
You can use tweezers to pull them off or glue the top closed on them but trying to get rid of them all is more harm then good. Id just leave them, they dont really hurt anything unless you have plague amounts. Watch out grabbing rocks though since you can cut your hand on them.
If you're going to leave them be, and don't want them to spread everywhere, limit your feeding. I have a HUGE problem with them mainly because I have a tendency to overfeed.