weird snails

[motox]

tonight i brought home a firefish. the lights have been out for about 1 hour. when i notice that he was not moving very much unit he swam out from under the rock were he was hiding and noticed that he had two hitch hikers stuck on him. when i went to net him to pick them off they let go and left a sore on either side of his back end. they were about 5mm long and they move very quick for a snail. the body of these creature are white and the shell is shiny cream colour. can any one tell me what they are are what to do about them. I think i just solved the mystery of the missing sand sifters that keep going missing