The blurriness might well hide the way his stare shifts, from Popp to the surroundings, as if this mysterious previously aggressively defended master might appear suddenly. That wouldn't go well. Public weakness was a thing to be punished by any teacher, and harshly. "Not like that. It's not allowed." An argument over whether or not magic was real was hardly worth it, and if Popp didn't manage to hide any such weakness quickly..

"You should stop before someone notices." Because THAT sure is a thing people can just switch on and off, surely.