The slime is one of the enemies in Redungeon. The slime looks like a green blob with a ghostly looking face (hard to see).
It move in patterns ,usually squares. If you run into it with no shields the slime will eat you. You can kill it with Sir Gylbard's shield or sword, or scare it to death with Creep A. Crow. While in the Duke's Darkwing mode, you can move through them without dying.