It's my understanding that Surge did in fact copyright Slender Man at some point in time (it's in one of the interviews, I'm not sure which one.) He keeps him as an open source character and seems to be fine with everyone out there making their own stories on the character, but so far all that has been produced is completly free to use. I mean sure, Marble Hornets has it's DVD, and several of the blogs and vlogs make money off of advertising, but a novel is new, and therefore in a gray area. My advice: better safe than sorry, try to contact Victor Surge and see if he's o.k. with this, you don't want to assume and get slapped with a lawsuit later. From what I've seen of him he seems fairly reasonable, though, at the very least he's okay with all of the other stuff out there, so hopefully it won't be much of an issue. Good luck either way.