This video was made by Unfiction user Magus for the contest seen in Self Defense. It was one of the winning entries and he got a response from the cast that was normal at first but then showed a scene with Slenderman. The cast seems to not know how it got in there as seen on the twitter page. The other winner apparently has not received anything yet. Note the video features the Operator symbol usually used in Marble Hornets and the "give me $20 dollars" song usually used in Slenderman spoof videos.