The cost would really determine what the actual defect or issue is. You can still contact Apple and see what they say. I would be interested to know the outcome. But otherwise, if you can't remove the stain with a microfiber cloth on the outside, I think it's under the display. And if it's under the display, that leads me to believe it's moisture. But then again, if you have owned the watch for over year and a half, I doubt the oleophobic coating would separate the way it is depicted in your pictures. Again, I am only speculating.