The elder McMichael is pretty much your generic-looking paunchy Georgia old white guy. Probably find a handful of lookalikes at any given grocery store. I too remain skeptical that this is a positive match based on this one photo.
I wouldn't be so certain that he would be well known simply from attending a KKK rally without concealing his face.
Efforts by antifascists to publicly out these kinds of people have really only picked up steam in the last few years, notably picking up after the Charlottesville attacks. A lot of their success can be attributed to the carelessness racists have taken towards protecting their identity.
A lot of these people who were doxxed were plainly showing their faces, and it only took a bit of social media hunting to find out who they were. These people covering their faces and taking more precaution is a more recent trend in response to the many successes of antifascists in getting people fired and otherwise ostracized.
It would be pretty bold for a law enforcement officer to openly march with the Klan, but stories of cops with links to the KKK are extremely common.
The ADL cities this photo from Rome, GA in 2016. That's all the way across the state from Brunswick. If this is McMichael, he could probably have been assured no one locally would recognize him.
https://www.adl.org/resources/backgrounders/loyal-white-knights-of-the-ku-klux-klan
The date is also pre-Unite the Right, when it was generally safer to be an out-and-out Klansman without word getting back to your boss.