tectus turbo snail dead?

[Nano]

Hi guys. Just picked up 2 turbo snails on Saturday, one was much larger then the other, upon putting them in the tank I notice the larger one was very lazy and actually rode around on the other one for most of the first day, the next few days he was fine going about his business. Now lastnight he was at the top of the tank on the gkass, and today is directly below where he was lastnight, his tissue looks live, but he hasn't moved all day... Is it possible he is dead or dying and why? My parameters are perfect. I had a salinity spike on Friday but had it under control before adding him so I doubt that would have anything to do with it. Any thoughts? I'm not sure if I should leave him for a while and wait and see could he crash my tank? The reason I ask if I should leave him is because I have an astrea snail that did the same thing and he's fine....