So, fun fact about the character of Slender Man: he is copyrighted, and naturally, Victor Surge (aka Eric Knudsen) is the copyright holder. Seeing as how he is not listed anywhere in the credits, I can only assume that the developers did not obtain permission to use the character in their game, hence them constantly referring to the titular antagonist as "The Faceless Man".

Other than that, it's just another Slender: The Eight Pages clone with the same jumpscare sound effect, a different map, a couple of interactive cutscenes, and for whatever reason, Blair Witch-style stick figures littered around the map. It does not earn the 6 dollar price tag at all, in my opinion, especially not with the two alternate modes, "Lost Memories" and "Hellmode". Spoilers on both modes below:

Lost Memories is the exact same game in the exact same map, with the only differences being that it seemingly takes place during daytime with a thick layer of fog, there is an extra line of dialogue once you collect all 8 notes, and there is a constant ambience of droning/banging noises which quickly becomes more annoying than unsettling.

Hellmode is the same game and same map yet again, with slightly less obnoxious ambience, and a thick red haze that makes it much more difficult to see. The only other difference is that once all 8 notes are collected, a deep laugh is heard before abruptly returning to the main menu.

There also does not seem to be any difference in the aggression of Not-Slender-Man across any of the 3 game modes, and its aggression doesn't seem that high in the first place. You could likely go an entire playthrough without so much as seeing it.

The one thing I can somewhat praise this game on is level design. It is very easy to get lost in the forest where you search for notes, which is definitely a trait you want in a Slender map. The one criticism that I have is that there are several invisible walls in places that otherwise seem completely accessible, and constantly running into them can get irritating.

tl;dr, it's a cheap attempt at cashing in on Slender Man's popularity 10 years too late.

It's a Slender man clone. Nothing more, nothing less. It adds nothing new.
I will say the graphics and sound design were great and it had promise, but overall just falls flat.
Cannot recommend.