Pareidolia

Pareidolia ) is a word from Ancient Greek to describe the phenomenon, that the human mind often perceives familiar patterns in a stimulus, even when they are not there. 
Common examples are perceived images of animals, faces, or objects in cloud formations, the "man in the moon", the "moon rabbit", and hidden messages within recorded music played in reverse or at higher- or lower-than-normal speeds.
Pareidolia is a special case of clustering illusion.