What is wrong with my Lettice Nudibranch? ):

[tonyydeee]

Hi! I've had this little guy/gal for several months so I'm thinking that he might just be at the end of his life, but just wanted to ask since I didn't know. He used to be big, dark green, very healthy looking. I've noticed the past week or so he's shrunken and turned very pale and yellow... What's going on? ):
thanks in advance. (Everything else / water is completely perfect)

http://i.imgur.com/c2rYmYI.jpg