I'm thinking maybe it's Brian or Alex who was wearing the mask, and they've somehow been caught by SM and are being held in that house by him.  Slender man seems to be an extremely quiet dude, so what i'm thinking is that when J finds that doll on the ground, and turns around to see the masked person, the person (brian/alex?) realizes it's J but sees the slender man coming out of that door to attack J, and tackles him to the ground to avoid being hit or something. Then the slender man does whatever it is that makes them both fall to the ground in a coughing fit, and attacks the masked guy and J, and does something to make J go back to his car and forget what happened.
Thoughts?
Also, i didn't notice slender man in the background of #16, that's some freaky shit. Someone mentioned something about him being in the bushes of the entry where they're in the car talking, but maybe i'm mistaken but i don't see him in number 7.