Your hand is no longer in contact with the rock, therefore there is not way for your hand to exert a force on the rock during its "flight". In that case, the only type of force left that could be exerted on the rock is gravitational force. Since you are on the moon, there is no air around you for friction (from the air) to act on the rock while in motion.
A common source of confusion for students about this topic is because tossing a rock results in a curved path, and it is strange to think that the force of gravity is the only thing acting on the rock in a curved path just like how the force of gravity is the only thing acting on a rock if you were to just let is fall straight down. In the curved path example, your hand is giving the rock initial velocities in the horizontal and upward directions. The horizontal velocity, since there is no force acting in the horizontal direction (gravitational force is an up/down force), will continue as-is for the entire duration of the flight, giving the path its parabolic shape. Compare this to if you were to toss a rock straight up - it would fall straight down without horizontal movement because there is no horizontal force to give it velocity in the side to side directions.
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