According to my investigations,there are some theory about slenderman(s).
1-Slenderman is not just one.There can be many of slendermans(Either male or female).
2-Slenderman can created in mind,and making your brain believe it.You can see,touch,hear it.But in your mind.
3-It can obtain its victims body and mind,making them suicide.(Maybe)
4-Slenderman has got 4 arms.Two are long as his chest.Two are little as nose.
Need more theory's.If you going to ask something,you can ask.