10:03 AM PT -- Looks like Bella was having a little fun with her latest beau, who actually turns out to be Blackbear disguised as DJ Marshmello, not 'mello himself.

While the all-white outfit and square-shaped helmet are typical for Marshmello, the ink on display definitely belongs to Blackbear.

Blackbear and Marshmello have a new collab due out soon and yesterday, Blackbear posted a pic on his Instagram of him wearing Marshmello's helmet.

Bella Thorne is no marshmello ... but she's dating one.

Bella and DJ Marshmello showed up at The Filmore in NYC Saturday night to watch Blackbear.

No one knows who Marshmello really is -- could be Scott Disick -- but she seemed into him .. at least for the night.

He blew up in 2015 when he released remixes of Zedd's "Beautiful Now," Jack U's "Where Are U Now" and other stuff as well.

As for Scott ... they seem to be hooking up, but on both ends it seems pretty non-exclusive.