Hexproof means a permanent or player cannot be the target of spells or abilities your opponents control (CR 702.11). The keyword applies at the moment something is being targeted — it does not retroactively invalidate permanents that are already on the battlefield attached to the creature.
Auras that are already attached remain so because the legality of an attachment is checked when the Aura enters the battlefield (CR 303.4). Once an Aura is attached, the game only asks whether it is still attached to a legal object via the 'Aura attached to an illegal object' state-based action (CR 704.5n), and hexproof does not make a permanent an illegal object for an already-attached Aura — only certain qualities like the wrong card type or protection do.
Protection is different from hexproof in this regard: protection causes attached Auras to fall off as a state-based action (CR 702.16e). Hexproof has no such rule, so an Aura already on the creature simply stays there.
Concrete example: Your opponent controls a creature enchanted with your Ethereal Armor. The creature then gains hexproof (say, via Swiftfoot Boots being equipped to it). Ethereal Armor remains attached and continues to pump the creature; hexproof only stops you from targeting that creature with new spells or abilities, not from keeping already-attached Auras in place.
Summary: Hexproof is a targeting restriction (CR 702.11a). It has no rule that removes permanents already attached to the hexproof creature, unlike protection (CR 702.16e). The Aura stays.
Unofficial fan resource — not affiliated with or endorsed by Wizards of the Coast. Answers are AI-generated estimates grounded in the Comprehensive Rules and are not a substitute for an official judge. Verify anything match-critical.