This is a good start; great artwork, good concept for gameplay, but there are some issues I noticed.
1. No matter what game you're making, you need to have an "Are you sure?" screen after selecting any quit option. This is vital to ensure the player doesn't accidentally quit the game; for example, in this game, it's possible that the player hits an extra button after failing and selects "Quit" without them noticing.
2. This either needs an option to swap to an alternate appearance of the cues or just change them to arrows, because mentally converting the letters to directions is just frustrating.
3. You should really move the target closer to the end/right, so you give the player the most time to prepare and can catch their misses asap. Currently, the entire right half is wasted be because the player can't do anything about the cues there, and it unnecessarily creates a potential for a delayed fail since the misses aren't counted until they travel the other half of the screen.
4. If you really want a challenge, try to figure out a way to prevent multiple cues from reaching the target at the same time; while it doesn't break the game when it happens, it is still annoying.