Mr. Green is a monster from the episode Substitute Creature, filling the role of Ms. Keane when she can't come to school that day.
At first, the girls think he's a bad monster and imagine him in scenarios where he does evil things to Ms. Keane or their classmates.
In reality he's a very nice monster that loves children and wouldn't harm them.
He has green skin, white hair, horns, and sharp teeth and claws. Wears an eyepatch, a suit with a cape, and has a scar under his eye.