They are called Endermen.
They are black, 3 blocks tall and look like a head with limbs.
They are based off Slenderman.
They move slowly when not attacking.
They are semi-hostile ...
... meaning they will not attack you unless you do something to them.
They can move blocks around, albeit randomly and (apparently) just casually when not hostile.
If you look DIRECTLY at an Enderman (as in put the reticule onto him) he will stop moving and turn slowly to face you.
You can put the reticule near the Enderman and use your real eyes to look at him without him becoming hostile.
AS SOON AS you move the reticule away, the Enderman runs at above-normal speed directly at you and attacks.
If you manage to get and look away, he will teleport once per second to a position inside your field of view and continue running and attacking.