The atmosphere was good, but I think that you could make the character die if Slendy catches him, because it has happened to me and there are no consequences. Also, it's too linear and repetitive, it would be a great improvement to add more level types, and switch between them more smoothly, you can't be in a neigbourhood and suddenly appear in the middle of a forest. I would add too a running motion (for example, holding spacebar while walking) to escape from Slendy, which would cost double the stamina that walking takes, but it would make escaping him and losing less sanity more effective. And it also would be risky, because as he can appear at any moment, if you are low in stamina you can't run and you would have to walk, and therefore, lose sanity.
Also, it would make a nice bonus to make a discouraging message appear in the screen every time you find a note, like "HE IS NEAR" or "PLEASE HELP Mnvnv" (nvnv symbolizes an irregular line, as if that person couldn't finish the message), or "YOU WILL DIE YOUNG". You know, sentences like that, to improve the uncanny feeling.
With all of the above, being a more complex game, I think it would be worth 4 or 5 stars. Anyway, good job!