Slugterra (2012)

S1E8 Deadweed

Eli is a skeptic when it comes to ghosts, but that changes once he steps into Deadweed Cavern. To his surprise, Eli discovers that his initial doubts were misplaced. What he encounters there is far more sinister than mere spirits; it's El Diablus Nacho, who has gotten his hands on the perilous Dark Water that Dr. Blakk employs to transform harmless slugs into Ghouls.