Mu Yanling, Sky Dancer — ruling 1

Mu Yanling's first ability can target a creature that doesn't have flying. The creature just gets -2/-0.

Mu Yanling, Sky Dancer — ruling 2

After Mu Yanling's first ability has resolved, the target creature will have flying again if another effect grants it flying.

Mu Yanling, Sky Dancer — ruling 3

An Island is a land with the subtype Island, not any land with a mana ability that produces blue mana.

Mu Yanling, Sky Dancer — ruling 4

Tapping an Island for mana is an activated ability. You can't activate that ability and also activate the ability it gains from Mu Yanling's emblem.

Mu Yanling, Sky Dancer — ruling 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.

Mu Yanling, Sky Dancer : règles, fonctionnement et interactions (MTG)

Mu Yanling, Sky Dancer is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

+2: Until your next turn, up to one target creature gets -2/-0 and loses flying. −3: Create a 4/4 blue Elemental Bird creature token with flying. −8: You get an emblem with "Islands you control have ': Draw a card.'"

How it works : Mu Yanling, Sky Dancer

Original English text — Wizards of the Coast / Scryfall

#1July 12, 2019· Wizards of the Coast
Mu Yanling's first ability can target a creature that doesn't have flying. The creature just gets -2/-0.
No other card is named in this ruling.
#2July 12, 2019· Wizards of the Coast
After Mu Yanling's first ability has resolved, the target creature will have flying again if another effect grants it flying.
No other card is named in this ruling.
#3July 12, 2019· Wizards of the Coast
An Island is a land with the subtype Island, not any land with a mana ability that produces blue mana.
No other card is named in this ruling.
#4July 12, 2019· Wizards of the Coast
Tapping an Island for mana is an activated ability. You can't activate that ability and also activate the ability it gains from Mu Yanling's emblem.
No other card is named in this ruling.
#5July 12, 2019· Wizards of the Coast
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.
No other card is named in this ruling.