Very unique take on the theme.
The gameplay idea of mixing rhythm games and shoot them up is very clever, and I would love to see this idea expanded further! Adding special types of enemies to kill with specific patterns of music could be fun! The drawing are also really great, and the story is a nice addition to fit the theme.
For the improvement, I would add a tutorial, as I didn't know at first that the ghosts could kill me.
And I have one question: why a piano piece for a guitarist? :o