If a leg has come off it's not looking good for him. It may be time to take him out of the tank if the condition continues. same thing happened with one I got. It pretty much disinegrated over a week or two until there wasnt much left of him.
I left mine in til he was just 2 legs and then I flushed him. :-(
Not sure how toxic he could be to other fish though. I didn't experience any spike while mine was dying but I may have gotten him out in time.