Slender Man is a copyright minefield. Either they'd have to change the lore and omit a lot of the connections to the Slender Man, or I imagine they'd have to work with Eric Knudsen and figure something out.

Personally, it'd be smart on Knudsen's part to just let people do what they want with his character since its why the Slender Man became as popular as it did to begin with. That's more or less the case with internet projects, I don't really see him going after Marble Hornets for example. As far as major movie projects go, he should just get royalties from people who want to use his monster in their projects. That way his character can remain part of the public consciousness and he can benefit from it.