Like most creatures on the Phlegm Planet, it is made of snot, and lives in its disgusting waters. Unusually, it has one eye, located on a stalk within the creature's massive jaws, instead of a tongue. Because of this, its jaws are always open when not eating, even when swimming, keeping its eye above the waterline and jaws ready to snap shut when prey gets close.
It normally keeps everything but its eye-stalk submerged while swimming back and forth in the river of snot at the bottom of the canyon, but if Earthworm Jim gets too close, it will try to bite him in half. Since Jim can't use his Plasma Blaster in this level, he has no way to attack Mucus Phlegm Brain - he can only avoid it.
- Mucus Phlegm Brain's name is a pun on mucous membrane, a substance that lines animals' body cavities and surrounds their internal organs.