If it's a YAM, the coupler is screwed on with teflon paste, which acts as a sealer/corrosion barrier. A special 3 or 5 finger tool is available, works best with an impact wrench . Threads are normal , right handed, in other words: turn to the left. You might try a block of hard wood against one of the coupler nibs and strike it with a hammer. In any case heat the coupler with a flame to soften the hardened sealer and expand the aluminum (alu. has a coefficient of almost twice that of steel- so this usually helps). I've had to saw them off before.
