Brian Austin Green Is Reportedly "Upset" Megan Fox "Moved On So Quickly"

Did Brian Austin Green Just Troll Megan Fox and Machine Gun Kelly?

The Instagram shade wasn't exactly subtle.

Brian Austin Green is reportedly unhappy that estranged wife Megan Fox has moved onto Machine Gun Kelly — and that she did so with social media fanfare.

A source told People the former 90210 star is "annoyed that she is so focused on her relationship with Machine Gun Kelly."

"He doesn't understand the need to post social media pictures with silly captions to proclaim her love for him," the source said. "They aren't even divorced yet."

Green, 47, seemingly made that discontent known when he appeared to troll Fox and her new boyfriend on Instagram last week. After Fox shared a photo of herself with Machine Gun Kelly (Colson Baker) with the caption "Achingly Beautiful Boy... My heart is yours," Green shared photos of each of his four sons (three of whom he shares with Fox) with the same caption.

People's source went on to say that Fox's social media behavior "definitely annoys Brian. And it stings a bit too that Megan moved on so quickly. Megan was always very private, so Brian just doesn't get what changed."

RELATED: Did Brian Austin Green Just Troll Megan Fox and Machine Gun Kelly?

Green previously confirmed he and Fox separated late last year. They married in 2010 after meeting in 2004 and getting engaged in 2006.

"It sucks when life changes and something that you’re used to, that you’ve been doing for 15 years, you try and not get rid of but you change," he said. "There’s the unknown aspect ... there's that pit in my stomach ... I really don't want Megan and I to be at odds ... she's been my best friend for 15 years and I don't want to lose that."

Earlier this month, he also discussed co-parenting in the wake of their split: "It’s not bad different, it’s just different. I wish Megan the absolute best in everything. I want her to be completely happy, for her, for the kids. That’s super important. No one wants to be around somebody that’s unhappy, nothing good comes out of that. That’s a terrible situation. We are taking it day by day."