Do a search for vermitid snails. My tanks has/had thousands of them. In some places there can be 20 in a square in of the live rock. Right now, I am looking at a trochus snail with about 40-50 growing on it's shell. A couple of times I took all the rock out and smashed all the ones I could find, but they soon come back.
I have heard that copperband butterflies might eat them. Another member on this board had great luck with dwarf zebra hermit crabs eating them, but I can't seem to find any of these crabs locally. They don't seem to cause any harm, but they are unsightly and can cut your hands when you work in the tank. Mine have also started to clog the plumbing. I wish you luck with this insideous pest.