Hi
The crescent moon status icon indicates that Do Not Disturb mode has been enabled on your Apple Watch:

Do Not Disturb mode prevents calls and other alerts from making sounds on your Apple Watch, whilst still enabling you to receive audible alerts for alarms and timers:
On your watch: when viewing the time, swipe up on the watch face to open Control Centre > tap the Do Not Disturb button (crescent moon icon) to toggle the mode on and off.
Your watch and iPhone can also be configured such that enabling Do Not Disturb mode on one device also enables it on the other. This mirroring option also applies to any scheduled operation of the mode on your iPhone (you may, for example, wish to set a nightly schedule that enables the mode overnight):
- On your iPhone, in the Watch app, go to: My Watch (tab) > General > Do Not Disturb - turn on Mirror iPhone.
- On your iPhone, go to: Settings > Do Not Disturb - turn on Scheduled and set your preferred times.
More information:
https://help.apple.com/watch/#/apd62807a9f3