This is what we found on Wikipedia regarding the Little Green:

Little green men is the stereotypical portrayal of extraterrestrials as little humanoids with green skin and sometimes antennae on their heads.

The term "little green men" came into popular usage in reference to aliens during the reports of flying saucers in the 1950s. In one classic case, the Kelly-Hopkinsville sighting in 1955, two rural Kentucky men described a supposed encounter with metallic-silver, somewhat humanoid-looking aliens no more than 4 feet (1.2 m) in height. Employing journalistic licence and deviating from the witnesses' accounts, The Evansville Courier used the term "little green men" in writing up the story. Other media then followed suit.