Bug milk

'''Bug Milk is a key ingredient in gaining access to the Nightosphere. It was seen in the episode "It Came from the Nightosphere," when Finn "reunited" Marceline with her father.'''

Appearance
The Bug Milk container appears to have two symbols on it: A ladybug near the top on the side, and what looks like a stool to be sat on while milking the bug. This may give a hint that a special kind of bug is milked to get bug milk.

Trivia

 * It is likely that Bug Milk is a reference to the 1988 comedy horror film Beetlejuice. 
 * It is unknown if it comes from bugs or not. 
 * If it does, it is unknown if it is to drink or if it has another function (Besides gaining access to the Nightosphere).