Cute game! But occasionally I'd move my mouse over the same note and it would play at a different pitch (especially with D), and I struggled to figure out how to actually place the note I wanted in the right place.
If you continue tinkering with it, it would be nice if the notes listed at the top were "grayed out" until you selected it so you could be sure you knew which one you were placing. There may have been other clues, and apologies if I overlooked them, but that visual would have helped me. Really really cute concept though!