He literally stands on the spot. He lowers his shoulder and braces for the contact and then runs. If anything aguero was fouled by the defender running into him.
More importantly Mustafi has nothing happen to him, Aguero is standing still, he bumps into him, barely. He moves about three inches from the contact and immediately stops playing, there was nothing there at all. You can't just run into someone and then get upset that you bounce off them if they are standing still. Blocking means stepping across the path a player is intending to take to purposefully block them, if you're standing still and someone walks into you, it's there fault not yours.
The basketball rules basically work here, player standing still gets barged it's a foul on him, player moves into the path of player with the ball and causes contact, it's his fault.
Except even there the contact to Mustafi is nothing at all, it's the slightest bump. Kompany put 10 times the force into Auba in the box in the first half, was never a foul either and Auba didn't make a single move to ask for a foul.