Yes, you can and should wash it. Still, rust forms worse, NOT in the areas you can see, but within the femder, door, rocker and quarter panel welds you do not. They form at the welds on the inside because the corrosion resistant metals in the the steel including galvanization is burned away during the welding process. Car makers don’t tell you this when they brag about their galvanized panels and great rust proofing coatings. That is why ALL cars are subject to rust eventually forming.
If rust stain settles on the exposed painted area you can see, and the paint or coating remains in tack, it is less of a problem then the salt in the brine that is washed down the window slots and up through the drin holes in your doors rockers and quarter panels. The fender liners may be plastic on newer cars anyway. The rust actually starts at the joinery between the plastic fender liner or steel liner and the fender. Those you cannot wash clean once the salt is inside.
Washing the undercarriage in a unibody car is really the only thing you can do but actually the least helpful considering the areas that will rust. The only way your car can remain in perfect shape is to either not drive it in winter, or treat the inner panels with oil. So obviously, your car was either treated or not driven much in the winter salt to last this long. Next time, it would be better to put it up earlier.