Little green men

"Little green men" was a general expression used typically by humans to refer to anything "alien".

The Eighth Doctor used it to refer to the Time Lords when he spoke about Penelope Gate. (PROSE: Seeing I)

After the Sycorax invasion of Earth in 2006, (TV: The Christmas Invasion) UNIT issued a press briefing, which, among other things, stated that "aliens are no longer little green men." (PROSE: Alien Life)

Lieutenant Stepashin used the phrase to stubbonly assert he didn't believe in the Eleventh Doctor's warning of an Ice Warrior. (TV: Cold War)

Little Green Men was a virtual reality game, created by the Etydion to turn humans into their kind in 2050. (TV: The Custodians)