Internal forces does not effect the motion of the body. When we mean motion of a body, we mean motion of its center of mass. And motion of center of mass occurs only when there is an external force to the system. Therefore in the case of tennis ball and coackroach there is no external force acting and the center of mass of the system does not displace, although there is relative motion between the ball and the coackroach.

 

Just like the Universe, there is motion of different bodies relative to each other but there is no motion of the Universe itself. If at all, then to where?