I think Slenderman does not have a face, BUT is capable of making others believe he does have one. This is similiar to the Noppera-Bo of Japanese lore, who "wipe" their faces off, revealing their faceless heads, in order to freak out their victims.
This would explain the original mythos belief that he has a face that just doesn't show up on film...he creates an illusion, but the illusion is only visible to the naked human eye...while Cameras don't photograph the illusion.