The poem starts out sounding like a firm statement: the soul is not a tiny man sitting inside us, working the controls from the heart or the head. It's the kind of thing a person says to dismiss a childish, cartoonish picture of what a soul is. Then comes the turn. "All right but who is that little guy" slips in a casual, almost cheeky question, as if the speaker has just caught himself mid-argument.
The joke is that the speaker can't quite shake the image he's just rejected. He says the little man isn't there, yet he's still imagining him, still curious about him, and even gives him a face of sorts. Saying "all right" sounds like a shrug, a reluctant giving-in to his own doubt.
The mood is light, playful and a little puzzled. The poem doesn't solve the mystery of what a soul is. It shows how hard it is to think about the idea without picturing someone inside us, and it lets us smile at that.