From the net, FYI to parents. (It does not seem to have anything to do with Harry Potter.):
Who is Slender Man?
Slender Man was initially the creation of a web user called Eric Knudsen. He entered a ‘paranormal pictures’ Photoshop contest on a comedy internet forum called Something Awful in June 2009. Participants had to turn ordinary photographs into creepy looking images, which were later passed off as authentic photos on a number of paranormal forums, reports Know Your Meme.
After Knudsen created Slender Man, describing him as a mysterious creature who stalked children, other Something Awful users began creating their own versions of Slender Man and these spread onto other sites, appearing in fan art, short stories, videos, video games and other media all over the web.
He is usually portrayed as a blank-faced, tall, thin figure in a suit, with tentacles protruding from his back. Depending on the interpretations of the myth, he can cause memory loss, paranoia and coughing fits (known as “slendersickness”), and can teleport at will.
Sounds like a bit like the aliens trying to pass themselves off as human in Whitley Streibers book.