Pretty neat! Just a few issues:
The biggest problem was the fact that slender doesn't walk at all. And when he drops in, it's slightly scary maybe the first time, but once you realize that he's just going to spawn in and stand and stare, it's not scary anymore because you can easily manipulate him.
One thing I miss from the original slender games was the fact that he'd be posted up at a distance a lot before you even encountered him, sowing fear into the players before actually hitting people with the real spook.
Player movement was also a bit much. You bob around a little too much, and you retain momentum more like you were on wheels as opposed to walking.
Other than that, we enjoyed it! Let me know if you update slender to be more threatening, then I might come back and play some more!