I recently bought a Aqua UV Advantage used from a seller that said it was only used for a couple of months. When I plugged it in to test out all of these gray and white flakes came out. I pulled it apart to check the bulb and the quartz sleeve and they both look fine. Any ideas what this is and how I can properly clean it all out?

Any fitting is an opportunity to spring a leak! — in some system design; minimum fittings is actually a priority!