Yellow snail

Hello everyone,

I have a mysterious problem with one of my cornu aspersum snails. It turned yellow some weeks ago and I can't find out why. I did discuss the matter in a german forum but we don't have a clue.
Google brought me here to an old post that describes my snail very well except for the yellow mucus. It is this one:
petsnails.proboards.com/thread/13437/bright-yellow-mucus-foot-disease

I'd be more than glad if someone here could help me.

Here are some pictures of my snail:
schnecken-forum.de/phpBB3/download/file.php?id=76647&t=1

schnecken-forum.de/phpBB3/download/file.php?id=76577&t=1

schnecken-forum.de/phpBB3/download/file.php?id=76611&t=1