Passive aggressive employee

I work at a small company where most of the employees are way older than me. I’m 24, and most of these employees are between the ages of 37-65. And I’m one of 3 managers, so I manage people way older than me who are more experienced. Anyways, there’s one employee who’s 52 and been doing this job a long time and his long time partner is the GM. So basically in charge of everyone. When I first started, I thought he was super chill and nice. But overtime I’ve seen him send extremely passive aggressive messages in our Slack channel about the stupidest stuff. But not a single person checks him. Because his girlfriend is the manager. He will single people out in the chat and start arguments with people, then end it with “idk just saying!” to basically put the blame back on someone else’s. I finally had enough of this and sent him a kind message back today about his concern, and was condescending to me and tried to embarrass me and make me look stupid. I genuinely cannot stand it anymore when it happens because it’s wrong, but I don’t know what to do about it. I value having a job and don’t want to lose it by standing up to him, and I feel guilty not standing up for others when he does it to other people, especially since I’m in management. What should I do? Everyone else just ignores him and just lets him carry on, should I do the same?