My first guess would be you've been hitting a drive through car wash. The valve stem is blocking the spinning sand grinder (I mean wash wheel, sorry) from reaching that part of the wheel. That, or your washing technique is younkeep folding the stem down against the wheel with your wash mitt instead of going around the stem. Try some type of cleaner wax or polish; the same thing you would use on paint and that should do the trick. Not anything overly aggressive, just a finishing polish. I'd try the cleaner wax first. Depending on your detailing skills, a cleaner wax would be safer. Also, don't be scared to wash the barrel of the wheel too. It'll do wonders for the appearance.