To start off:

IF you don't know anything about the slenderman it's a creature that lurks in the forest and when you sopt it you will be hunted down by the thing. It stands 2 times as tall as the normal man and as many victims say, it has NO face. Here is a explanation video of the urban legend

Now WHY is the enderman so slimilar to the slenderman and what makes it similar to it you may ask?

Because it is rather tall and has hands nearly to the ground. Because it stares the victim down until it will attack and you can't run or hude from it JUST like the slenderman chasing down it's victims sooner or later. And the fact that it just moves slowly and creeps around the place. And lastly the name Slenderman with out the Sl at the front, it will spell enderman

MAYBE Jeb got the inspiration from the slenderman or it has nothing to do with it but we can never know until we get the answer and if so, why would jeb ad this creature to the game. Maybe just for fun or something more.
We might wait until the answer will be spoken :l