After you play Moonfall, if you play a unit to that battlefield after, will it also have -2?

No, a unit played at or moved to a battlefield after Moonfall has finished resolving will not receive the -2 Might penalty.

According to RiftJudge FAQ #10053 and #10722, the Might reduction effect of Moonfall "snapshots" the game state at the moment of resolution. It applies its modification only to the enemy units present at the targeted battlefield at that specific moment. Any units that arrive at the battlefield subsequently—whether via the Ambush keyword, a standard move, or any other method—are not affected by that specific casting of Moonfall.