Does Moonfall's ability to reduce all enemy units might by 2 at a battlefield affect units played after Moonfall resolves?

No, Moonfall does not affect units played at that battlefield after it resolves.

According to RiftJudge FAQ #6445, might reduction effects like the one on Moonfall "snapshot" the game state when they resolve. They apply their modification only to the units currently in play at the targeted location at the moment of resolution.

How it works:

1. Resolution: When Moonfall resolves, it gives all enemy units currently at that battlefield -2 Might for the turn.
2. Snapshot: The effect then stops checking the game state.
3. New Units: Any unit played at (or moved to) that battlefield after Moonfall has finished resolving will not receive the -2 Might penalty.

This behavior is consistent with other similar cards (like Thousand-Tailed Watcher or Smokescreen), which apply their debuff once and then cease to affect the battlefield further.