Barnacle Man is an alter-ego/version of Barnacle Boy in the episode "Mermaid Man and Barnacle Boy V."
Barnacle Man is basically Barnacle Boy dressed in a vampire-style cloak with a red shirt that has a skull-and-crossbones on it. He is part of EVIL (Every Villain Is Lemons).
Role in episode
Barnacle Man is formed when Barnacle Boy joins the dark side after getting fed up with Mermaid Man
still treating him like a child.