Not Marines: Marines would almost certainly be in uniform. Authorizing the wear of civvies while on any sort of duty is rare and absurd for a situation like this. Further note, the guy on the far right is not wearing an undershirt... IF this was somehow authorized they’d certainly be following all regulations on proper wear of civilian attire.

Additionally, those guys are at neither attention nor parade rest so they aren’t following drill manual.

Can’t speak to Secret Service but based my limited experience with those guys I’d expect them to follow a higher standard too.

I’d bet it’s private security.