I do not see a small moon anywhere.
Its in the screenshot you posted, top right corner.
Hi David. That moon is an indicator that the phone’s Do Not Disturb feature is turned on.
For me with android 11 (Samsung Galaxy A51) the do not disturb icon is like a no entry sign, circle with a bar through it and it is displayed on the left side of the screen.
I have always associated the moon symbol as relating to day and night. I don't use Dark mode on any of my computers or devices.