'Slenderman' is apparently now a copyrighted character. However, the real slendermen are still about in various forms, sometimes called by different things depending on myths and cultures (tall greys, shadowmen, etc).
The former was sold & marketed to make money which lost their scariness, the latter are still pretty much scary as heck imo.
And is somehow starting to get away with it. That's why he's secretive about who he's given the rights to (management or sold, whatever). If people find out, he'll lose his hold...& probably be sued by those being barred from using SM imagery/mythologies.
I think he managed to get his personal creation story (the whatever style mythology) tied in with the Slenderman figure & its myths as to be indistinguishable from each other.
Right now there are companies out there who believe he owns the rights. We need more info on which parts he owns (as in, just a location, a certain story, etc), if any, and separate the two - one Public Domain (as it should be), the other Fan Art (his myth addition).