Must be a shelless snail, or maybe even a nudibranch! Any color to it? Watch what it likes to eat,carefully. Things may look really cool but can be a disaster or, it might not be. I did have a large shelless snail that used to move up the glass but it would eventually take the shape of an ovoid, or oval, with a tail the same semi opaque color. Must say I love getting unidentifiable critters coming out of rock, however it rarely happens to me! Also had a chiton in the beginning, but haven't seen it in ages. That really is disappointing. Such a primordal looking critter.