The Halloween quest only unlocks when you play one of the specific Halloween games (i played Halloween sugar rush for it) but it does not unlock when you play horror games, or zombie games. Even though the quest says it should unlock on "scary" games.
@leedade I believe this due to the tags (or categories). If you check here and search for "scary" or "Halloween", you will see that they are two actual tags/categories of games. So, if the game is not in the "scary" or "Halloween" category, it probably doesn't give the quest.
I just won this quest about an hour ago. I searched for "scary" games under the all categories header and then picked a random scary game. I don't remember which game it was because I left as soon as I was awarded the quest.
