What I would consider primarily is keeping the background and foreground elements separate. Use lower contrast elements for the ground and high contrast for the ships, projectiles, etc. One strategy that many shoot em ups use is making darker backgrounds (a lot of this may be due to space settings, but the design and the theming go hand and hand) and lighter ships, to further the separation. This can work in many other ways (dark vs light, saturated vs desaturated, complimentary colors, etc).
At the moment it's very difficult to pick out the ship in your mockup, and this is due to the value (light/dark) of the ship being very close to that of the ground, and the ground being very busy. For the background, focus more on larger areas of solid color, rather than small clusters of texture. It's easier on the eyes and makes scanning the scene for things like bullets, enemies, and yourself a lot easier.