Does an aura attached to a permanent with hexproof fall off when hexproof is gained after the aura is already attached?

Hexproof only prevents a permanent from being targeted by spells or abilities controlled by opponents (CR 702.11). It does not retroactively invalidate Auras that are already legally attached. Once an Aura has resolved and attached to a permanent, it remains attached unless a game rule or effect specifically moves or removes it.

The rule that checks whether an Aura's enchant condition is still met is the 'illegal attachment' rule (CR 303.4c). This rule causes an Aura to fall off only if the Aura's enchant ability no longer permits it to be attached — for example, if the permanent gains protection from the Aura's color. Gaining hexproof does not make an existing attachment illegal; hexproof restricts targeting, not what can be enchanted.

Auras only require a legal target when they are being cast (CR 601.2c) or when they enter the battlefield and must be attached (CR 303.4). After that, the question of targeting is moot — the Aura is simply attached, not targeting anything.

Concrete example: Your opponent controls a Pacifism enchanting your Grizzly Bears. You cast Ranger's Guile giving Grizzly Bears hexproof until end of turn. Pacifism stays attached and Grizzly Bears still cannot attack or block. Hexproof didn't change whether the Aura's enchant condition is met.

The only way an existing Aura falls off due to hexproof is indirectly — for example, if a permanent gains protection (not hexproof) from the Aura's color, because protection causes the Aura to be removed (CR 702.16e). Hexproof alone carries no such effect.