Setup:

  
I chose to follow the guide in the GG wiki's Guide: Moonlord Strategies:

Worm Scarf (I went to another world to fight the boss but could have just grown a corruption here)  
Master Ninja Gear

Frozen Shield

Charm of Myths

Shiny Stone (this is why it has to be AFK. Shiny Stone only works when not attacking)

Star Veil

Brain of Confusion

Armor is Valhalla Armor with headpiece replaced with Squire Helm.

Potions include Lifeforce, Endurance, Ironskin, Regeneration, the heart-gathering one, and Summoning (since summons were doing all damage).

Damage was done by Ballista Staff and Stardust Dragon.

Weapon held was Brand of the Inferno, whose right-click adds 20 defense when a shield is equipped and which doesn't interfere with Shiny Stone.

Arena included Bast Statue, Campfire, Heart Lantern, Dryad, Gnome, and honey blocks, as well as a 1-second timer attached to 4 heart statues and a skeleton statue for i-frames (I was originally going to get a Ham Bat and attack skeletons with it for even more regen but decided on Shiny Stone instead).

Had to check this for the wiki, but it's still possible to AFK facetank the Moonlord with a defense/regen build