Good one! You can say it's a sort of rhythm game, but I think this could be the starting point of a whole new genre, haha.
My thoughts on the game: The core mechanic been done great. It's simple, solid, straight to the point and serve the purpose very well. As a software it's a good “Instruction program”, but from a video game perspective, the game play part feels a little bit weak.
I think adding some gaming elements could make it much better. Such as leveling, perks, unlocks, etc. Even just a visible progression bar could help to make player feel every stroke have some feedback just like normal inputs in other type of games.
But that's just my personal opinion.
As the first of its own genre this game stands on a great starting point and has great potential, hope it could keep growing.