Does hexproof prevent a Pacifism already attached to a creature from remaining on it?

Hexproof means the permanent "can't be the target of spells or abilities your opponents control" (CR 702.11b). It is a targeting restriction that applies at the moment a spell is cast or an ability is activated — it does not retroactively invalidate targets that were already legal when the spell resolved.

When Pacifism (or any Aura) resolves, it attaches to the creature as part of resolution (CR 303.4). From that point on, the Aura is simply a permanent on the battlefield attached to its enchanted creature. The game only checks legality of existing Aura attachment if the rules specifically call for it — namely, if the Aura's enchant condition is no longer met or the creature has protection (CR 704.5n). Hexproof does not appear on that list.

Protection is the keyword that does cause attached Auras to fall off (CR 702.16e), because protection prevents the permanent from being "enchanted" by sources with the specified quality. Hexproof grants no such benefit to already-attached permanents.

Concrete example: Your opponent's Grizzly Bears is enchanted with your Pacifism. Your opponent then casts Slippery Bogle's text onto it (giving it hexproof). Pacifism stays attached; the Bears remain unable to attack or block. The hexproof only stops you from casting a new targeting spell or activating a targeting ability against the Bears.

In summary: hexproof (CR 702.11) stops targeting, not existing attachments. Only protection (CR 702.16) actively removes Auras mid-game through the state-based action in CR 704.5n.