Honestly, I'm not sure how you'd really get Slenderman to work in a movie to begin with? I didn't watch Marble Hornets myself (I am a wimp who can only deal with playing some horror games and otherwise just reads way more horror movie summaries than they should), but I had friends who were very into it. My outside observation is that part of why that worked was the tension over not knowing what the next part would be, and there also being "real" time over which the story can develop, right?
And, well, beyond that, isn't the whole point of Slenderman that he's just sorta... there, in the background, when you don't expect it? That's harder to do in a way that shocks the audience when you're making them sit down to watch a long movie vs being able to obsess over a single several minute long YouTube video for a while...
My thought was always that it'd be interesting to see it executed as a somewhat short game that has him following you, and then at the end of it suddenly replays your entire recorded playthrough from his perspective (or else just from an angle that shows how he was there the entire time), or something along those lines.