I have a dented white patch on my butt - (like on my lower back)

I don't know what it is. Its not vitiligo..or whatever its called because it's not just white..but dented. When I say dented - you can feel the skin make a crater - Does any body know what this is? It white, dented, patch (quater sizes), it doesnt itch or hurt - but it looks terrible. I also have eczema...but I don't think thats related. Please help. I don't know what else to do..

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