Looking After Snail with Broken Shell

[marym]

Hi. My name is Mary and I’m in the U.K. I found a garden snail 9 days ago with a broken shell. I’ve been keeping him in a container with veggies, leaves and cuttlefish. The snail has spent most of the time on the underside of the lid. I’ve been misting the snail and container twice a day with bottled water. This morning, I noticed a soft, whiteish bubble on the snail’s left side. This is the first time I’ve kept a snail so any advice would be appreciated. 
I would like to attach a photo from my iPad but couldn’t work out how.