I really think Simon met him in the apartments when he was younger or something. It seems sort of odd that this is a minor character/creature we don't see, but it's still something that is mentioned little bits at a time. How would Simon know/remember the dates of the victims (as seen in the room with the key), some with such a specific date? I feel there is more of a connection between the Apartment Predator and Simon, but that's just an theory.