Geryuganshoop was one of the Alien Invaders who attacked A-City.

Geryuganshoop is an alien with a face that consists of nothing more than three holes, two being eyes and one being a mouth. Its body is entirely black with little to no detail. It has no arms or legs, but rather a multitude of tentacle-like appendages. In anime, he appears to possess blue body covered with translucent skin, which turns rainbow-colored in fight.

(Source: One-Punch Man Wikia)