Three short lines, and a whole transformation happens in them. The speaker has been drinking rum late into the night and doesn't feel well. The poem's answer to that misery is surprisingly light: "I turn moth." There's no long explanation, just the change itself, stated as plainly as if it were the next step in the evening.
The title, "Metamorphosis," nudges us to think of a big, dramatic change, but what we get is small and rather humble. A moth is a night creature, drawn to lamps, a bit clumsy and dazed in the dark. That fits a person who is up too late, woozy, and drifting around in the small hours. The speaker seems to feel less like a human being and more like something fluttery and fragile, pulled along by instinct.
The mood is quiet, a little queasy, and oddly tender. There's some dark humor in it, but also a sense of loneliness and giving in to the night.