squealing radiator belt

There are no additives that can be used on flat serpentine belts. They will make dirt stick to the belt and cause even more squealing. The belt needs to be replaced now and the pulleys must be scrubbed to remove any soap residue.

There are two potential causes of squealing. A pulley is tipped or turned causing the belt to walk across it as it goes around it. That is most commonly caused by an idler pulley with bad bearings. The belt could also be loose. Most commonly that is caused by a spring-loaded tensioner pulley that is rusted tight.

instead of replacing it why cant i just wash it off?

When belts get warm dressings or coatings soak in and can never be washed out. You can try a spray-on engine degreaser that gets washed off after a few minutes but no professional would ever risk making a customer angry with a repeat failure. If that works on your belt, that's fine but most customers get angrier at having to come back to the shop a second time than they do if the mechanic just replaced the belt right away. Trying to save customers some money always ends up biting us later. Then we get an undeserved reputation for doing poor-quality work.