The Crab Demon is a character in the series Adventure Time who appears in the episode "Red Starved." He and Paris were planning to turn a city under the Desert Lands into sand and reanimate the inhabitants to conquer his homeland at the center of the planet, but this plan fails because he zones out for long enough for his partner to die. His third eye can shine a ray of light that turns anything (except gems) it touches to sand. Finn encounters him while looking for something red for Marceline to eat.
His skin is light green, and he has a darker green shell on his back. There is a third eye on his forehead. The sclera of the third eye is pink, and the iris is red. (Although, since at the time the Crab Demon is seen from Finn's colorblind perspective, there's an excellent chance his eye is actually a shade of green.)