If you're seeing things which only YOU see and others don't see I recommend going to a doctor, no joke. If you are also playing the 'Slenderman' video game a bit too much, you are going to see him in your dreams frequently because dreams are memories of the previous day being sorted. I would suggest cutting down on the time spent gaming and remember it's YOUR dream, if you want to will him to go away, he will. As for the encounters in real life, if you're not getting enough rest you could be hallucinating or it could just be tricks of the mind. I hope i helped, Just inbox me if you have any questions.