I never heard the term or knew who or what slender man is or was (till I looked it up recently; literally this morning...) but my old House on 327 Brittany Rd. had one... (lived there from 1999-2007)

he was at the end of the hall but something was different about mine...

He occasionally wore a black wide brimmed hat...

but he wore no face but an occasional menacing grin... an unsightly grin...

he haunted not only the hall but the storage, back door, my room, the large bedroom, and bathroom... and would on RARE occasions be seen looking out my bedroom window and loitering about outside standing there... silently watching...

My Mom on occasions would be visited by him... as he stood at the doorway, the closet... or even lying there next to her in her bed...

while I never left my room at night for he stood right at my bedside... his cloak like figure obstructing the view of light...

in truth I believe he is a Poltergeist... and draws from what emotions you have then builds around such to taunt in the most psychologically disturbing ways...