Little green men were a specific science fiction conception of extraterrestrials on pre-first contact Earth. They often traveled in flying saucers.
Captain John Christopher, after having transported aboard the USS Enterprise in 1969, claimed to have not believed in little green men. His statement, which was overheard by Spock, approached and startled Christopher, stating that he didn't believe in them either.
Later, after Captain James T. Kirk was captured for trespassing, he explained to Colonel Fellini that he was, in fact, "a little green man from Alpha Centauri." (TOS: "Tomorrow is Yesterday")
Following Rain Robinson's discovery of the yet unknown USS Voyager in 1996, she indiscreetly spread the word of the contact, which eventually reached a Cal Tech professor. That professor, in turn, contacted Henry Starling, who said of the contact, that "we're not quite at the little green men stage yet." (VOY: "Future's End")