Very nice. I wasn't able to figure out how to respond properly to the rocks, but the rest was very intuitive. I would recommend making it so that when the player hits a bubble note, they are also challenged to release the button in question in time with the bubble reaching a certain minimum size. Add that together with some sort of scoring or level or health system (like a small bar that drops drastically when a note is missed and rises slightly when a note is hit), and this could improve dramatically.