Here's the deal ... the Green Day rocker was performing in D.C. this week, and while he was onstage -- somebody gave a full Trump mask to him, which had the word "IDIOT" scribbled across the forehead ... which is an homage to another mask BJA once donned back in the day.

Fans might remember this well ... during the Bush years, Billie did something incredibly similar -- rocking a Dubya mask with the same word. Now, it was Trump's turn ... and as we know, Billie ain't a big fan of DT.

It's unclear if Billie ended up wearing the whole mask, but one thing he did do was hold it up in front of the audience while he was singing -- and that image is now going viral for all the wrong reasons.

As you might imagine ... conservatives on X and elsewhere are circulating the pic of Billie holding up the Trump mask -- and suggesting it looks kinda like he's holding up a Trump "head," as it were. Clearly, that's not what he's doing ... but that's where folks are taking it.

As a result ... Billie's getting dragged for being insensitive, especially in light of the recent assassination attempt.

The drama is reminiscent of Kathy Griffin's 2017 saga, where she found herself in hot water for posing for a pic with a fake bloody head that looked like Trump. KG was read the riot act for the photo scandal ... losing several brand deals and gigs in the aftermath of the drama.

While she did apologize for uploading the picture, her attorney later defended the image ... saying it was simply a work of artistic expression ... and didn't mean any real harm.

The key difference between Billie Joe and Kathy's controversies ... BJA simply held up a mask of Trump, and in no way condoned or encouraged violence against the ex-Prez.