there is no specific side mission in Grand Theft Auto V (GTA V) where Michael has vivid hallucinations of aliens while interacting with an activist. It is possible that this side mission was added in a later update or a mod, or it may be a misunderstanding or misinterpretation of a different mission in the game.
In the original version of GTA V, Michael, Franklin, and Trevor are the three main protagonists, and each has their own unique abilities and perspectives. While there are missions involving aliens and hallucinations in the game, they are not specific to a side mission involving an activist.
If you are referring to a mod or a custom-made mission, then the behavior of the characters and their experiences can vary greatly depending on the creator's imagination and intentions. Mods can introduce new storylines, elements, and interactions that are not present in the original game.
In any case, without more specific information about the mission or mod you are referring to, it's challenging to provide a precise explanation for the differing experiences of Michael and Franklin. If you can provide more details or context, I'd be happy to try and help clarify.