Sea hare dying? or typical behavior

I purchased a sea hare last Friday. It seemed to be doing a great job at first. Over the last few nights when I check on it, it is not really moving around and then I noticed this increasing large white area on its back. Tonight when I came to check on it, it seemed to be breathing heavily. There is still lots of algae for it to be eating, so I know that it is not starving. Any ideas. I have removed it and placed it in a separate container in my sump. I do not want it dying in my tank and expelling its poison. Is it possible that it is just sleeping when I am checking it, or has it slowly been dying and tonight is likely its last night.