Okay, since when does Mario have bleach-white skin and multiple stretchy arms?

That's clearly not Mario, since he's standing there to the right of the screen in just his boxers and undershirt. Slenderman is simply an A+ dickwad who stole Mario's clothes and is attempting to steal his name as well. He clearly has no idea who Bowser, King Koopa is or that Peach is a princess, not a queen. So not only is Slendy a dickwad, he's a moron.

BTW, Slendy, that's not a fire flower you're holding. That's a fucking piranha plant you swiped from a pipe. It's getting pretty mad at you and wants to bite your ass.