Try locking up the mirror and using a blower, everyone should own a blower anyway. If its still there, read up on wet cleaning and decide if you want to do it yourself. Sometimes the dust is sticky, pollen perhaps, and you must wet clean. It's not really hard at all. As long as you have a steady hand and some sense of what would be appropriate pressure. Read up on it though. Don't use a cotton swab. Don't ever touch the mirror. They sell the proper sensor swabs and cleaning fluid. I'm cheap and make my own swabs from a Pec pad. I only end up doing it twice a year or so.