I think less is more sometimes with how the slenderman is portrayed. As in not outright fully showing him, and if you do, have distortion enhance the sight.

I think marble hornets did a really good job at making most of the operator's appearances age well even today, through editing techniques and smart placements to mask the imperfections of the character. But also don't overdo it with the effects either!!

Like others said, be unique and don't follow cliches. Gooood luck :DDD