Can soldiers in civilian clothes salute a fallen comrade memorial?

I salute the flag when the anthem plays and when we recite the pledge. Nobody has ever said that I couldn't or even questioned me. I think it would go to our first amendment protections but even so, we've earned the right.

A person not uniformed of which you know not their past or present military affiliation can make whatever gesture with their hands as they desire...

When my grandfather, a WWII Army veteran and who was well loved and respected by many was laid to rest, he was given a military funeral and when Taps was played, myself, my brother, our step dad and every other person there ( there was a large contingent from our VFW post of which he was a long-time commander) who were veterans, none of whom were in uniform and without any prior conversation about it, popped tall and rendered the hand salute and there wasn’t a rule, reg, or sumbitch big enough to stop a single one of us from doing it. I think it’s ingrained in us from our AD days and it’s the highest honor we can give to each other in those moments. So, I say, if it strikes you as the thing to do, then do it, and if anyone has the audacity to say anything about it afterwards, refer them here and I’m sure that there will be a long line of us who will set them straight.

I pity the fool that tries to stop me from doing so. As they opened a can of whoop ass and I will do my damn best to whoop their ass.

Always! Once a vet, always a vet. Our service transcends uniform protocol.

My father passed away July 2021 he had been a Marine and I never had a chance to salute him while he was alive. I did not hesitate to salute him at the cemetery. I wanted to honor my father so I did. I have since found that it is alright to do so but at the time I really didn't care it felt right so I did so.

Yes, under President Obama, law was passed that allows all military and Veterans to do so, specifically Saluting the Flag

Lol. Who is stop him? A SGM?

Unless they recently changed something . Veterans have the option to salute in all situations. I know and do at every opportunity to render honor. We render honor when the flag is raised, the National Anthem is play and at Memorials to our fallen brothers and sisters .

As the question asks "Can," I would emphatically say yes. There is nothing which prevents anyone from rendering such a token of respect, nor is there anything mandating it.