What does the blue half moon mean?

The moon icon indicates that Do Not Disturb mode has been enabled.

To turn it off: when viewing the time on your watch, swipe up on the watch face > swipe left/right to the Settings glance > tap on the Do Not Disturb button (which has the same moon icon).

When turned on, Do Not Disturb prevents calls and alerts (except for alarms) from making sounds or lighting up the screen on your watch.

Also check your settings, where you can control whether activating Do Not Disturb mode on your iPhone also activates the mode on your watch - and vice versa.

On your iPhone, in the Watch app, go to : My Watch > General > Do Not Disturb > turn the setting for Mirror iPhone on or off, as preferred.

You're welcome! 🙂