Almost positive they are vermetid snails. They don't move, they build the shell around them. Best way I've found to get rid of them is to super glue over the end. Stir up a little of your sand, you should see something resembling a web comeing from the mouth of the stem that the sand (dust) will stick to. That web is how they feed, like sending out a fishing line that catches food. They can irritate coral, and as you have seen, reproduce like crazy.