Pareidolia is a term from psychology that refers to the tendency in people to perceive a meaningful pattern in a vague stimulus.
For example, look at the moon. Some people claim to see a man in the moon. That's pareidolia. Psychologists note that once your brain has assigned a meaningful pattern to something, it holds onto that interpretation. But there's no face on the moon.
East Indian people see a rabbit on the moon; Samoans say it's a woman weaving; Chinese people see a monkey pounding rice. Because of the previous assignment of a different meaningful pattern, it's as hard for a Chinese person to see the face at first as for you to see the monkey.
Any time you see a person in a wallpaper stain, a ship in the clouds, or a face on Mars, you're experiencing pareidolia.