hey there guys, I was cleaning my MacBook pro's screen with a very lightly damp cloth, and drying it off with another completely dry microfiber cloth, like I always do. But when I was working on the lower left corner of my screen this weird stain appeared. The more I tried to wipe it off, the larger it go. Does anyone know what it is or how to fix it, or whether the apple warranty covers it. It doesn't affect the actual screen at all, it is just very annoying as I try to keep my MacBook in a pristine condition as much as possible. Thanks ! Its a late 2013 MacBook pro retina, if that helps.