From SpongePedia, the First SpongeBob Wiki.
Mermaid Man and Barnacle Boy. He's only seen in the TV shows. Kelp Thing's first and only appearance so far was in Mermaid Man and Barnacle Boy VI: The Motion Picture. His only known power is flying.
In the "real" Mermaid Man and Barnacle Boy movie he wanted steal the Krabby Patties from the Patty Vault.
Kelp Thing wears a green suit, a yellow necklace, belt, gloves, boots, and has eyes like a snail.