He can;t do anything about his host body's mouth.
If comparing to real life, his body was speaking MTL (Machine Translated Language), even though the context were very different...
Also, i know he did it on purpose because he value the original flow of the story while enduring the harshness of what Harold will experience in the future.
MC knows that he really needs to ready himself to absorb all of that to remain in control to prevent his death.
However, a slap from a chick like Erika hurts my heart. MC can endure the pain on his face, but talking about the heart i hope he will not mentally breakdown...