Update: Back when I made my post in late June, I called AppleCare to complain about the defective anodization on my space gray sport watch and they sent me a replacement Apple Watch Sport space gray body. Fast-forward a couple months, and the same thing with the anodization started happening again.
I agree with JayLenochiniMac that this problem looks like defective anodization. The best thing I can think of is that during manufacture, the case is left with a sharp edge from the tool that cuts through the case where the digital crown comes out, and sharp edges are known to
leave gaps in the anodized layer because the anodization grows outward in flat planes, "The anodic layer grows outward from the aluminum surface as a part is anodized. As such, very sharp inside or outside corners on a part can cause gaps or voids to develop- especially with thick anodic layers such as Type III hardcoat anodizing. Avoid very sharp inside or outside corners when possible, and allow a radius of at least .015"(1/64th")." It seems like the aluminum starts to corrode from this edge underneath the anodization, possibly gaining access to the aluminum through a defect in the anodized coating at this edge. This effect is probably worse with the space gray case for any number of reasons, since the anodization is much darker than the metal underneath, and possibly with the space gray just being less durable, with all the photos of the anodization
flaking off the back. I should also say that I don't handle any caustic chemicals or do anything strange with the watch. It just gets exposed to air and water and a little errant soap from handwashing from time to time, which makes this issue all the more egregious. Also, if this is an anodization issue, you would expect to never see this kind of issue with the stainless steel Apple Watch, which I think is true.
So after this happened again with the new watch body (along with my peeling black sport band, of which I am on my third, currently), I called AppleCare back. They then showed some photos I took of the case to the engineers, and it was deemed that this constituted "cosmetic damage" and would not be covered. No one at Apple bothered to even physically inspect the case before issuing that decree. It turns out that the first time they replaced the body, it was done simply as a one-time courtesy, and no one at Apple bothered to physically look at that case, either.
With all the marketing that Apple did to highlight the aesthetics of the device, how this was the most personal device they had ever made, the high price, and the fact that this is a first generation product, Apple should stop hiding behind their old "cosmetic issues are not covered" policies developed for computers. The aesthetics of a watch are a lot more important than a desktop Mac, so the policies should be different for cosmetic issues. They are going to lose customers if they keep telling people that yes, your watch looks bad (and YOU must be to blame for it, there's no such thing as an "Apple-caused cosmetic defect"), but no, we are not going to do anything to help you.