({G/U} can be paid with either {G} or {U}.)
Enchant creature
When Shielding Plax enters the battlefield, draw a card.
Enchanted creature can't be the target of spells or abilities your opponents control.
If the target creature is an illegal target by the time Shielding Plax tries to resolve, the Aura spell doesn't resolve. It won't enter the battlefield, so its enters-the-battlefield ability won't trigger.
This is not the same as hexproof. If, for example, you target one of your opponent's creatures, your opponents won't be able to target their own creature with spells or abilities.