They do like to bury themself I think, mine buried it's face into the gravel once, it was kinda cute/funny. I think he was trying to hide though....
Anyway, he could have been sleeping or their could be a problem with the water quality.
Mine has some kind of white stuff floating on his foot, but he's hasn't acted weird, just sleeping and climbing around and under and over and about the tank he is in. lol
I think it's just something that got stuck to him.
Maybe that's true for yours too?
I think Apple Snail.net is a good place to go to find info on the snails and caring for sick ones.