Thanks everyone for the replies!
I have a quick update: since yesterday the "withe sack" is disappeared, it went smaller and smaller everyday and, the worm disappeared too with the "white sack". The snail is active and go around and dig his head in the gravel looking for food.
My idea is that this worm was (probably is, somewhere) a detritus worm and he was eating the stuff that the snail got stuck on the side of her head digging with it in the ground. What do you think?