Do units played after Moonfall resolution still get -2 might?

No, units played at or moved to that battlefield after Moonfall has finished resolving do not receive the -2 Might penalty.

According to RiftJudge FAQ #10053, might reduction effects like the one on Moonfall "snapshot" the game state when they resolve. The effect applies only to the enemy units present at the targeted battlefield at the exact moment of resolution. Any units that arrive at the battlefield subsequently are not affected.