The player should replay the Time for Adventure mission. This time, the final Green Power Star is located on the Millstone Planet. The player needs to travel through the millstone planet until he or she reaches the part where the Power Star is. The player should see the Green Star off to the distance. The player must climb onto a millstone to head towards the star. Once the player is close enough, he or she should perform a somersault followed by a Star Spin to reach the Star.

I think I've found something that may help you : http://www.mariowiki.com/Clockwork_Ruins_Galaxy#Green_Star_3Source

"The player should replay the Time for Adventure mission. This time, the final Green Power Star is located on the Millstone Planet. The player needs to travel through the millstone planet until he or she reaches the part where the Power Star is. The player should see the Green Star off to the distance. The player must climb onto a millstone to head towards the star. Once the player is close enough, he or she should perform a somersault followed by a Star Spin to reach the Star. "

Maybe you should try it out, see if it helps you, good luck!