That adjuster looks pretty rusty, you might want to considering ordering new ones as they are relatively cheap and easy to replace. Last time I did my drum brakes both adjusters were seized and the square drive strips super easily. Also as a tip, one of the easiest ways to adjust it is using 12 pt 1/4 drive 7/16 socket and flipping it around to use on a 3/8 inch drive ratchet.