Megan Fox just went scorched Earth on her ex, Brian Austin Green, for posting a Halloween photo with their youngest son ... and she didn't pull any punches.

BAG threw up the post on IG Sunday, which depicted him at home in a Yeti costume and his and Meg's boy, Journey, behind him in what appears to be a Spider-Man outfit. Brian's doing some goofy pose, and he captioned it ... "Hope you all had a good Halloween!"

Innocent enough, right? WRONG!!!! At least according to Megan, who left one hell of a public comment ... ripping BAG to shreds.

She writes, "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." Megan adds, "I had a great halloween with them yesterday, and yet notice how absent they are from my social media."

She continued, saying ... "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. You have them half of the time. Congratulations you truly are a remarkable human!"

One last jab from Megan ... "Why do you need the internet to echo back to you what should be inexhaustibly evident in the way your children love you?" Oof, gut punch. Then again ... Brian has posted his three boys' faces before, so it's strange MF flipped over this.

We've reached out to Brian for comment ... so far, no word back, and we're guessing he's not gonna respond.