The social media war erupted on Sunday, after Green shared a photo of him and 4-year-old child Journey -- who he shares with Fox -- on Instagram. He captioned it, "Hope you all had a good Halloween," but Fox was not happy.
"Why does Journey have to be in this picture? It's not hard to crop them out. Or choose photos that they aren't in," she wrote in the comments. "I had a great halloween with them yesterday, and yet notice how absent they are from my social media. I know you love your kids. But I don't know why you can't stop using them to posture via Instagram."
"You're so intoxicated with feeding the pervasive narrative that I'm an absent mother, and you are the perennial, eternally dedicated dad of the year," she continued. "You have them half of the time. Congratulations you truly are a remarkable human! Why do you need the internet to echo back to you what should be inexhaustibly evident in the way your children love you?"
He didn't reply to her directly, but did re-upload the picture, this time cropping out Journey.
Marcil, who has aired a lot of her own issues with Green on social media, appeared to show solidarity with Fox on her own Instagram page. On Sunday, following Megan's comment, she posted a screenshot of the Notes app reading, "Like I said....The truth always comes out in the end...."
In her caption, Marcil simply wrote the hashtag, #ImWithYouSister.
Marcil, who shares son Kassius with Green and dated him from 1999-2003, recently slammed him on social media in September. While the post wasn't actually about her ex, one of her followers commented: "I think you still love Brian Austin Green." That sent her into a posting spree.
"I was never in love with him but I am very proud of the fact that my son loves his dad unconditionally and has chosen to forgive," she wrote. "I celebrate Kassius' love for his father and remain extremely grateful that Kass was finally invited back into his father's life and was able to meet his youngest brother for the first time & finally start seeing his other two siblings again. 5 years apart with no explanation is devastating for children. For siblings. My son is also an incredible & protective big brother."
Marcil previously claimed Kassius had been "cut out" of Green's life while he was married to Megan Fox. The actor never publicly responded to her claims, but he began sharing more social posts with his son shortly after being called out.
After Green uploaded his cropped photo to Instagram this weekend, someone jumped into his comments to write, "Let's see all the Halloween pics with his first kid ... oh ya there aren't any." In response, he simply said, "there was no social media then."