What you are describing is a very common chemical reaction that occurs when steel is exposed to water, and especially in salty water. The problem is that steel contains a lot of iron, which will eventually form rust in a humid environment.

You're quite correct that galvanized steel is supposed to prevent it from rusting. However, over time, even this protective layer will erode away, exposing the iron underneath.

The rust, or brown and red foam is just iron oxide. The fact that some of it has appeared both on the surface of the water and at the bottom of the container just means that some of the rust has been knocked off from the steel surface. I bet if you looked closely, you would see some rust forming on the steel mesh as well.

The good news is that I wouldn't expect that the presence of a small amount of rust should affect the growth rates of the calcium and magnesium carbonates.