The secret ingredient of slurm is revealed in S02E06 “Fry and the Slurm Factory”. If you don’t want it spoiled (it is a major plot point of the episode, but not really essential to the series as a whole), you should rather watch it. If you just want the answer, here it is. Slurm is provided by a colossal slug/worm hybrid’s (or “slurm’s” if you will) heinie. It’s kinda like a cow’s milk or a bee’s honey. This is a single component of Slurm.
See yourself as a character in the famous cartoon