Great to see you're having fun while experimenting!
The "circle below moon" you mention is the sun/moon shining through the terrain.
To avoid this you need to enable "receive shadows from surfaces" in the atmosphere node.
This takes a LOT longer to render so I STRONGLY suggest to perform a crop render of the troubling part of your image and paste it into your original work in (say) Photoshop.
To get more moon-rays you need the following:
1) A "dusty"/misty atmosphere which allows the light to be scattered to allow glowing rays.
2) Something which casts shadows to form/create the rays. You can let the terrain do that by clever positioning of the sun/moon or to use clouds.
It's quite challenging to do that actually, so I wouldn't bother too much about it yet
Cheers,
Martin