That green growth is algae and is not a good thing, as it could quickly cause root rot, or even clog your pump and misters. Your rez should be completely light proof to prevent algae growth; spray paint it black a couple of times, cover it with mylar/reflective insulation. To answer your question about changing the rez, it should be every week. Another thing you are going to have to watch is the temp of your water. You need to keep it low (68-75), if you can. When you get ready to change your rez, you are going to realize a simple design flaw in your system (I know I made plenty before I made one that I liked, so dont feel bad buddy). By having your rez and plants in the same container you are going to have a mess when you change your rez.
1. How are you going to drain?
2. What are you going to do with your plants while you are working in the rez?
This is why most people maintain a rez that is separate from there growing containers. Anyway buddy, its not the end of the world, your just gonna have to work a little bit more when you change it out. Make sure while you are doing this grow that you keep a notebook and write down all of the problems that you run into and keep track of when you planted so you can work out timelines and maturation dates. This way when you go to do your second grow, you will have a wealth of information at your disposal. Anyway, fire away with any questions you have.