First things first, great game! reminds me of devil daggers and is probably why I love it so much.
I have 3 complaints that if fixed, could make this game into a "full release"
1. There is basically no progression, it does seem to get slightly more difficult over time, but you get bored faster, after 500 seconds i started to get bored of the repetitive maneuvers so I challenged myself to stay very close to the middle. Perhaps add more types of ghosts that move differently and have special mechanics.
2 . The sound. I usually say that sound makes or breaks a game. If you work on the sound some more to create the perfect vibe it will make your game hundreds of times better!
3. The atmosphere doesn't quite fit with the game, since these are ghosts I would imagine it would be a more creepy and dark environment, having the game darker will also add more of a challenge.. or perhaps have it slowly get darker and darker as time goes by (this would help with point 1)
Please don't take my critiques negatively, I'm only critiquing to help you learn as a game developer, also these are just my personal opinions and may not necessarily fit the game you have imagined.
Either way, I love the game so far, keep up the good work!
Also if you don't mind check out my game too, I'd appreciate it a lot! :D