I saw nothing wrong with what Muller did.
He got a backhanded slap in the face - those hurt at that velocity. 99% of players will drop down in that situation to emphasise the foul, given you are pretty much taken out of the play situation by such a foul anyway.
Then Pepe goes and butts heads - I think it is to Muller's credit that he jumps up and gets in his face rather than die a Rivaldo-esque death from it. If he were playacting, that would surely have been the way to go.
I think he got hurt for a moment, Pepe loses his rags and Muller actually increases his chances of staying on by jumping to his feet rather than dramatizing the much lesser, but legally more severe, contact. If the ref was unsure of what he saw, he might have let Pepe get away with that given that Muller does quite the opposite of playacting on that.