In Markiplier's recent Powerwash Simulator video, he claims that the reason Slender Man has no face is because he (Mark) powerwashed it off. I have no evidence to prove he wasn't claiming this as literal fact.
It's a joke
It all makes sense now
...Don't let nightmind hear about this...
Just in case it needs to be said: There is the context of Mark being a funny guy playing video games online.
Which is to say it is rather unlikely he genuinely thinks he washed the face of an ancient humanoid abomination.
What do you think?