It is not a snail it is a Lichen, most are harmless I have tons in my 75 keep your eyes open I bet there is more. Also check around the rim of the tank I bet you'll find a bunch of empty shells of them there stuck in the salt deposits.
Are you sure your looking at the right thing?? Its the big thing in the middle of the pic? Its about the size of a quarter? Kinda Purplish colored Not the little white sqigly things. Just checkin! Todd
Its not that i didnt believe you, I was just makin sure. And ive only got 2 of them, have had them about 6 months. They came in o some Live rock i got from a guy who was getting out of his swf. tank. Anyway i googled it and it must be mispelled, either way At least i dont think there harmfull, and they move really slow! Thanx for lookin and ill find the right spelling to it later and read up on them. Thanx for your time! Todd