SF Symbol
I've had a look and can't find it either, but it might not be an SF Symbol at all. It may just be an icon they've created.
If you want to create it yourself, start with circle.dotted, fill in one side of the circle, and add a semicircle.
Where did you see this symbol ? In an Apple's app or another app ?
I searched also for circle and for dotted. Could not find it either.
moon.circle is relatively close to this. But not the same.
Recommended
This is a custom symbol that is unfortunately not available in the SF Symbol library at this time.