"Little Green Men" is a phrase often used when jokingly referring to intelligent beings from outer space. It is a popular notion that flying saucers are crewed by little green men, and the abbreviation LGM is commonly used by those discussing the topic.
LGM doesn't seem to be commonly used as an abbreviation for a singular little green man though. Perhaps single seater UFOs are piloted by BEMs or BHMs and the LGM seek safety in numbers?
LGM, or Little Green Men, is also the name of three piece rock band from Dallas/Fort Worth.