Unknown Green Guy
It is likely that he is one of the many incidental super-heroes seen in an all out confrontation with Madame Majestrix and her minions. But in that double-page panel image, there is no complete certainty of it being the same individual.
There has been some speculation among readers that he could possibly be The Cometeer. It's based on the fact that The Cometeer has never been seen and was first name dropped during the Local Heroes stories, which is in the same time this green man first appeared. But other than having an alien like appearance, nothing else in his appearance of a trench coat would match that.
Though if it is him in Astro City: The Dark Age Book One #4, it shows him with the ability of flight and in space like jumpsuit, which could lead credence to him being the Cometeer.