Glop is a translucent, light blue beverage that the crew of The Christa drinks.
In "Nowhere Man", Harlan accidentally spills glop on the engine, causing the power surge that makes him invisible. They spill glop on it a second time to recreate the accident and bring him back to this dimension.
It is possible that glop is a universal drink. The "ancient alien soda" Harlan finds in "King of the Hil" is the same blue hue as glop, but it might not be the same beverage.