The FOX announcers mentioned that Alfred Morris is very humble and doesn't like to celebrate too much, but that the celebration was actually a way for him to salute a little league team who requested that Alfred Morris do something for them. (Read more from the Redskins Blog)
After he hits the ball and watches it fly, he throws up a Three Finger Salute in honor of his College Football team the Florida Atlantic University (FAU) Owls. The salute is what they call "Owl Fingers"
Check out the GIF of the celebration: