I would really hesitate to paint inside the back wall. Most paints could break down over prolonged exposure to the saltwater. Why not keep the paint on the outside, or tape a black background to the outside of the back wall. Then if you ever change your mind or move the tank, all options are still open. Yes you will have
algae growth on the back glass. However, unless the paint is some anti-bacterial type (that would really kill the tank), you will have algae grow over the paint as well. I use a commercial plastic sheet intended for backgrounds. It is deep blue on one side and black on the other. I started with the blue and switched to the black which I much prefer. Changing or repairing the paint would be a major pain. Good luck with it.