There needs to be a slender man version of killing floor where he will wander the map as an unkillable entity. If someone is out and alone and sees him then he follows them, if anyone else sees him he will branch into two characters and he will follow both of them. Eventually when he catches whoever he follows that person dies.
He can follow the patty but only when he is not cloaked.
He can kill the patty (Just to show that no one beats slenderman.)
He can not follow basic zombies because they have no minds of their own. The doctor is in control of himself the zombies are programmed.
(Could you imagine slenders following ALL the zeds? WHAAAT?!)