Does hexproof prevent the opponent from targeting a creature with auras they already control on it?

Hexproof means a permanent cannot be the target of spells or abilities your opponents control (CR 702.11b). The key word is target — hexproof only matters at the moment a target is being chosen, such as when a spell is cast or an activated/triggered ability is put on the stack.

An Aura that is already attached to a creature is not targeting anything. It is simply an enchantment with an ongoing effect. The targeting happened once — when the Aura spell was originally cast and placed on the stack. After it resolved and became attached, there is no further targeting occurring (CR 303.4).

If a creature gains hexproof after an opponent's Aura is already attached to it, the Aura does not fall off and does not stop functioning. Hexproof gives no mechanism to remove already-attached permanents.

Example: Your opponent controls a Pacifism attached to your Grizzly Bears. You give Grizzly Bears hexproof with Ranger's Guile. Pacifism remains attached and continues to restrict the Bears — hexproof did nothing to dislodge it, because Pacifism is no longer targeting anything.

Note: Hexproof would prevent your opponent from casting a new Aura targeting your creature, or from using an ability like Auratouched Mage that re-targets the creature, since those involve choosing the creature as a target at that moment.