Who, or what the Slender Man is is quite a mystery. Many theories, like the Tulpa Effect and the Quantum Theory are good, so I though I'd share my theory with you guys. Please, this is MY theory, so don't start saying in the comments that this is wrong and it should be this or that, and that this is ridiculous. And don't say that he doesn't exist, because I think he does, and if you don't, then fair enough. By the way, if this theory already exists, then sorry, I didn't know. I just thought about it!
I think that the Slender Man is actually some sort of ghost of one of Victor Surge's relatives. Think about it. Did Slender Man just mean to pick Victor Surge as his first viewer? And, if you look carefully, Slender Man actually knows that the photographer is there, and is actually either greeting him, or getting ready to unleash his big hug of death (sorry, couldn't help myself!). He is also quite close to the photographer, and in other photos, he's more in the background. Maybe he killed the photographer, then left Victor Surge the pictures. It's just a theory, though, so what speicies Slender Man is is still a mystery.