Sometimes we base our identities on things that really have very little to do with who we are. We can do this to ourselves, but more often than not it's really the culture that we live in that calls the shots here. For example, we tend to identify people and judge their value based on their occupations. We don't know what kind of job our main character in "The Man-Moth" might have (probably used to make tons of cash as a sideshow on Coney Island), but we do know that he has an obsession. That obsession consumes him to the point that he can no longer separate himself from it, and so he bases his whole identity on it.
The Man-Moth is really the super-charged alter-ego of the Man mentioned in the first stanza. Minds: blown.
The Man-Moth's identity is completely informed by his obsession to reach the moon. Without this desire, he would be lost—like one of those moths flitting hopelessly around a light bulb.