Roky Erickson was in The 13th Floor Elevators, one of the first psychedelic bands from the 60s. After a few years of constant touring, he was involuntarily incarcerated in Rusk State Hospital in East Texas, where he received forced thorazine doses and electroshock therapy. From my perspective, he was a misunderstood genius ahead of his time, and was unfairly punished for it. When he got out, he formed Roky Erickson & The Aliens and made The Evil One, full of songs with horror related themes. I highly recommend this album.
In some ancient cultures, life is represented by three phases of the moon waxing (youth), full (adulthood), and waning (old-age). “The moon to the left of me” refers to the wisdom of old age. So maybe the ghosts in the song have everything, including the ultimate knowledge, since they passed on. That is just my own interpretation mixed with some other’s. Either way, it is a great song.