For a car driven only one day a week here is no good way to prevent disc rotor rusting (unless you can move to Phoenix). Rotors rust quickly – that’s what they do.
Generally this is normal and harmless. A few applications of the brakes will remove this light surface rust. Of course, the rust will only come back. Did someone tell you that you needed new rotors because of a little rust? Next time, don’t be so hasty to replace them. Take a short drive and come back with shiny surfaces.