It took me a couple of runs before I realised that I wasn't supposed to destroy the green ships. Some sort of hint or tutorial about this would have been appreciated.

I think I hit a bug, every time I played it just ends when I get a score of 2.

Design wise, it was a little hard to see your bullets and the ships against the ground. there was maybe a little too much noise on the screen. Maybe having the bullets glow slightly, and have some hit effect/glow on the ships to help display that you're hitting them would have helped also. 

Seems to be a bug where progress is halted and the game ends with a score of 2 despite no ships reaching the middle point.