When we were kids, my sisters and I used to make jokes about how much information a character could communicate through a "knowing glance." When we made up stories, we would often assign too much meaning to our characters' wordless expressions, goading the others to call us out on it.
However, in that second-to-last panel, that expression communicates one concept with 100% clarity: "Stop talking!" Decker understand this.
He just doesn't agree with it.
