Those cable guides are brazed onto the steel frame, and not removable. I suspect that water wicked between the cabl and fitting, and also un the uhderside against the frame, and stayed there to start a rust process.

You need to attend to this before the rust on the tubing gets any worse.

Remove the rear brake cable, and use sand paper, steel wool and/or a chemical rust remover to clean the affected areas down to clean bright metal. Then paint or use a clear paint to make it nier (depending on your aesthetic tolerance.

If you don't attend to this, the rust will eat into the tube at the braze on like a cancer, in which case the frame is toast.

One other option if the fittings are compromised is to carefully cut/file them off, and clean it up down to the tube. Then you can use cable ties to hold brake cables on.