Mu Yanling’s last ability affects only creatures you control with flying at the time it resolves. Creatures you begin to control later in the turn (or creatures that that gain flying later in the turn) won’t get +5/+5.

In a multiplayer game, if you leave the game after Mu Yanling’s first ability resolves but before your next turn begins, its effect lasts until your next turn would have begun. It neither expires immediately nor lasts indefinitely.