Looks to me like you might have gotten a little zinc in there somehow. Even minute amounts of zinc will give lead castings an oatmeal like consistency.
Tacshooter
If the slag is hard, Tacshooter has it nailed; zinc.
If it is goopy or soft, the other guys have the answer.
If zinc, there is no way to reclaim it, and you may have ruined your pot.
If it's glue you can flux and scrape.
Larry