In reply to Simon.

@Simon - You are asking several questions in one there. If you were subject to someone taking remote control of your machine and attempting to open a specially crafted GarageBand file, your machine has already been compromised to allow such remote access. A factory reset would allow some system files to persist that may be related to how an attacker was able to install a RAT in the first place. A full format and system scan is your best bet at this point. Thanks for your comment!