It might sound like a good idea at first because you're out of mozzarella and pizza sauce and pizza crust but don't do it. The cheddar is going to end up REALLY weird and it won't taste right. You can use spaghetti sauce instead of pizza sauce if you don't mind that it's sweeter but you can NOT substitute mozzarella for cheddar on a pizza. ESPECIALLY DO NOT USE SLICES OF AMERICAN CHEESE ON PIZZA.