"Earths spaceship is lost in space with no fuel"
Totally missed that! Sorry! Consider then maybe a trail of some sort?
About the deleting moons controls, I'm not really sure. It just felt weird to me. Like, I'd expect that moving moons was with the left mouse button, and deleting with the right one. But then, adding moons could get messy. Maybe use something like CTRL? I'm not sure. I'd say ask people that work with software that does similar things (like artists and such) what they usually use for those functions. It's something really minor, so I'm not saying that you should go out of your way to change it.
when I found something wrong I had to change it for every single level
This is really concerning! I don't want to know how you're structuring your code and levels, hahaha