It was intentional handball. The bad thing is that just a second earlier Messi clearly handled the ball too, couldn't control it and neither could Muller who also handled it.
Unfortunately for Muller, Messi had his back towards the ref and he didn't. The ref saw Muller's and didn't see Messi's hand ball. If he did the action would have been stopped there and Muller wasn't gonna get a yellow.