FWIW this coral pictured is in an isolated tank now (the pictures were taken a day after it was moved into a 20g - the only thing it shares the tank with are an abalone, some cowries and the patch of zoos you see - which I threw in the tank a couple of weeks ago as a sort of canary to let me know if it's cycled or not). It might be too late though.
I actually gave up on the idea of ever putting SPS into my 75g. This coral was put into a separate system, which is why the same symptoms really surprised me. I was really hoping for a different outcome. Let's face it, my ritteri tank is actually quite void of diversity. 3 fish, 2 clams (well, 4, 2 are being babysat), and then some GSP that hitchhiked on a rock one of the clams is attached to.
But, I recently moved a Platygyra out of the 75g into the 115g because it was receding in the 75g, in the hopes to stem that loss. And there are plenty of other things that I do (ie. use the same turkey baster to feed), that for a pathogen would make for a convenient pathway to cross-contaminate the tanks. I know I'm reaching on this, but I do really wonder if there's something to this.
"Fail" is such a strong term BTW. I don't know if I consider my tanks as completely failed even if they can't support SPS. There are plenty of other things about them that aren't all doom and gloom. There's just.... "room for improvement" shall we say. Ok, no, I guess they're partially failed.
Sigh. I did call this thread "heartbreak" for a reason.