The picture is fake sadly so Slender Man was not on Gravity Falls.
Slender Man is a specter or a ghost that is believed to haunt many southern cities. He has been spotted by ghost hunters in places like North Carolina and South Carolina, but there is no way to tell exactly where the ghost lives.
Slender man
Slender
...Seriously? Cuz he's Slender, meaning skinny, thin, etc.
slender man but in girl-form
the slender man
slender man is real
It necessarily. Slender man only exists as an existential being that can appear only when he is known. Or in simple terms-only if you believe he is real. There have been multiple modern day sightings of the being
Slender Man.
skinny = dünn, mager.
When you happen to see Slender Man, simply picture him as calamari or as somebody who is virtually senseless to overcome your fear. Remember, too, that Slender Man is not real.