Well, that's no dirt, likely, but it's loosing the color, so you can't clean the corner's.
At the moment, the official HTC facebook support page is the best place to change our phone. So far, there is still no statement from HTC about this issue. There are more and more people worldwide with the same "dirty corner" problem, because it's painted the color only on the black cover comes to the fore. Some people report the support will tell you to change to a black 8X, because they have no problems (without colour, it'ts logic).