Looks like the outermost, dead layer of bark has fallen of and the next layer is not yet bleached by the sun

For the most part, I think Hug's got it. There are a few pathogenic processes where you can end up with something like. Look up slime flux and bacterial wetwood, two of the finest terms in all horticulture. Good eye, noticing these subtleties-it's a big part of what makes trees and all of nature so enticing. But I'll bet there's no problem here.

Hug/Tom's right. Sometimes nut-hatches or squirrels will chip/chew away outer bark in a large patch looking for insects and the result is a patch less weathered-gray than the rest. Or it just gets so loose it falls off. Nothing of concern.

I've had a red oak have Slime flux, it is a frothy wet sugary patch, it went away anyway. These pics show dry bark, although some is new, the guys above said it all.

Thanks everyone for the answers. I had speculated it was just going through normal changes but my father seemed adament that it was a disease. Thanks for the reassurance!