Close-up of damaged leaves. I hope this is not verticillium.

Doesn't look very promising. Take a sample from the wilted portion to your local cooperative extension office for a proper diagnosis but Verticillium wilt is distinct possibility.

What's all the white dots on the soil? Flower petals?

Yikes! It looks like ammonium sulfate granules.
Mike

I don't know but I would probably prune away the diseased branch in case it's spreading inwards

If it's verticillium, pruning exterior portions doesn't stop the disease. It's internal - a disruption of the vascular system. But pruning off the wilted portion and taking to the extension service for a proper diagnosis is advised. And sterilize your pruners afterwards.

To diagnose verticillium from the staining seen on a sample, the removed (failing or very recently dead) branch should be at least the diameter of a thumb.

I suspect this was verticillium, but I was not able to send a sample to confirm that. I pruned back significantly below the dead area and luckily the tree survived!

I had problems with my Autumn moon as well. It grew very nicely for about 5 years, then two winters ago the entire right side of the tree died. The left side looked abysmal, but I decided I would try to even it out by selective pruning over the next few years, but it completely died over this past winter. I have a nice 'Koto no ito' in a pot that I might put there. I like the screening from the neighbors!