Every Star has its Price
If you complete Fluffy Bluff Galaxy, and then you go back, there is a hidden star. Get the cloud upgrade and go to the big tree. Jump as high as you can before you spin, causing a cloud to appear. Use all three and you should be able to get to the top of the tree. There is a small lauch star, and the yellow Toad that is always sleeping. Use that launch star and you should be on a cloud and you should see a blue Luma. The wands he holds have coins on them instead of star bits, telling you that he wants coins. You need 100 coins. It is possible to get 100 without moving on to the next area. Either by picking them up, bouncing on enemies, or getting one the two big coins.
|A Hidden Star||Get 100 coins and feed the Luma|