If it's not scratched it could just be a buildup of gunk on the screen. You could try using isopropyl alcohol (rubbing alcohol) to clean it off. Just make sure to remove the battery first and use 100% IPA if you can find it (the lesser stuff has water in it and increases the chance of a short if water seeps inside and doesn't have a chance to evaporate before reinstalling the battery). There's always the risk that IPA can damage the screen if there's any kind of special coating on the screen, but I've used it on mine with no ill effects.