It's a dual moon phase

Dual moon phase -- quoting from the original HW PR

"Creating a celestial stage for the lunar mechanism, the lower part of the dial features an elliptical opening, with two moons crafted in aventurine for a subtle mixture of spangled and sequined deep blue. Each moon – one white, one black -- is finished with a comet’s tail trace, set in shaded tones of diamonds and black sapphires. The whimsical mechanism is wound using a flush push button, located at 2 o’clock."

The schtick is that the black trace -- visible in he HW press photo below -- is the "dark side of the moon"