Fanfiction is a story written by a fan on an already existing character, (as PastaMonsters is not a written work it would technically be a fanCOMIC) The character Slenderman is not a written work in any of his forms.
This looks something I would see in Portal 2. You know, the ratman drawings? I really like this one and I fear him. THis guy made me see the dark differently. One of the few picture that didn't scare the crap out of me.