Newton's First Law of Motion.

In an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by a force.

Since there's no atmosphere or wind resistance, the flag is very sensitive to movement. While setting the flag, the flag was set in motion. So, it'll keep moving. Eventually, the movement will die down due to friction with the pole and the (much lower) gravity, but this would take much longer than on Earth where wind resistance will quickly stop the flag's motion.