I think this is right place, so my pond snail Sheldon has been being quarantined for about a month, without problems. He was very active, constantly eating the algae on the substrate and walls. I got my Betta yesterday, and ever since then Sheldon has been barely moving. Is he afraid? My betta (Beep) is curious about him though.
Pick your snail up and smell him; check to see if he'll retract. It's not normal for them to be that extended out of their shell. Know that a new tank does not have algae; usually it takes quite a while for it to develop. If Sheldon is dead, unless you supplement fed, he could have starved to death.