His text says: "Forced Response: After a player plays a card, until the end of the phase that player can only play cards that belong to a different faction than that card."
Scenario 1) A player plays a Hastur card, may he then play a neutral card? A neutral card doesn't belong to a faction so I assume he may not.
Scenario 2) A player plays a neutral card first, he then may play a Hastur card?
Conclusion: If a player wants to play a neutral card he has to play it as his first card. Seems like an ugly card to treat mono faction decks.