When I started playing it felt very clean and nicely polished. I really like the character design and the projectiles sound very crisp. The pixel art is very well done, too. I'm a sucker for small details and I definitely appreciated the flowing water from the first level.
As I played the game more I felt there were some missing polish areas. There isn't enough audio or visual feedback when you're hit or you hit an enemy. I think the character should flip directions if you're holding right click and pointing behind the character. I really liked that when you killed an enemy you saw the blue stuff get absorbed into the character, though a death animation of some sort would have created a better overall effect. When the character dies you're instantly brought back to the main menu as well.
I would also encourage you to try to add in a bit more of your own unique ideas into the game. It felt too similar to the classic metroid games for it to really stand out for me.
Overall, solid work!