The greenish [color=limegreen]x[/color] mark is the coordinate of the center of mass for those two objects.

The center of mass is moving at constant velocity, either before or after the collision.
Because there is no extra force acting on the system (system with the above two objects).

Xcm= m1*x1+m2*x2/(m1+m2);

If you switch to [b]cm[/b](selection box): observer at center of mass , you will find the greenish [color=limegreen]x[/color] mark will not move.