The Party Worm is a character with no speaking role in the episode "Evicted!" He attends Finn and Jake's party when they find a new home in a cave. He leaves with all the other partygoers when Marceline comes to reclaim her home.
The Party Worm looks like most of the Worm People in the show, except his skin is shades of brown instead of green. It makes him look remotely like an earthworm in real life, even though he also has a bulbous head.