The video showing the alien on the moon was posted by YouTube user Wowforreel and viewed more than two million times in less than a month. He figured out an irregularly shaped dark spot on Google Moon.
According to NASA scientists, the image was taken by either the Apollo 15 or 17 manned missions to the moon in 1971 and 1972, respectively.
"My best guess, is that something (dust, an eyelash, scratch on the negative) was on the film," Pix11 quoted a scientist as saying.
"Remember, this was in the pre-digital days when all sorts of nasty things could happen to film," he added.
Why the video went viral can be explained by a psychological phenomenon called Pareidolia. This is the brain's response to seeing faces and other significant objects in random stimulus, media reports said.