Even if Eric Knudsen will give his blessing to Slendercoin, there still would be third-party trademark holders, so I don't see a way to clarify the situation. But, unlike Mr. West, Slenderman in a fictional character, and no one should be morally abused or offended by using Slenderman in some open-source program. And, despite the fact that Slenderman was created by Eric Knudsen and was copyrighted by some sided holders, it's a part of folklore, just like Sandman or Cthulhu. And in that point of view, there should be no barriers to use it (just do it artistically and with roleplaying, like the "Offerings to Chtulhu" does, lol).