Thing with mud when it gets packed in your rad is that the heat from the rad dries it out and it becomes like concrete. You could use a pressure washer, though you need to make absolutely certain you don’t damage the cooling fins. What I would do is soften the mud up by watering it down, letting it sit a bit, then repeating the process till you think it’s softened up. It should then come off with your garden hose once it is. And welcome to the Forum.