Reading your first comment I spotted the figure before opening the very heplfull image you posted =) But of course I have no way to prove it.

Well, seriously speaking, I guess human brain has a rather developed capabilities of face recognition. Which makes us spot faces in nearly random shapes as well. I think even small babies react to the basic smiley-image, just because it has dots for eyes and a smiling mouth. It is the same here; shadows and shapes of the forest offer enough recognizable figures for the human face recognition to be triggered.

But I really don't know why you are scared =) To me the face in the forest seems like a friendly one.