The new Twitter chief showed up Monday night to the Moxy Hotel in NYC, where Klum was hosting her 21st annual gala of getups. He was dressed as some sort of warrior with pretty intricate crimson armor.

His mom was there too ... Maye Musk showed up at one point near Elon, dressed in goth-themed garb. A lot of eyes were on them, without a doubt -- the Twitter buy-out is top of mind these days, and EM's stunts have thrust him to the front of many conversations.

Anyway, the rest of the costumes were pretty sweet too ... including the one worn by the host. She was transformed from head to toe (and even beyond) as a fishing hook worm, with incredible attention to detail ... right down to the ribbing along the body.

Her husband, Tom Kaulitz, was dressed as a fisherman ... and Heidi was his oversized bait. At one point, though, she lost the worm suit and showed off her figure underneath.

Other stars who showed up dressed to impress -- Ice-T and Coco, Heather Graham, Julia Fox, Chloe Kim, Antoni Porowski, Jonathan Van Ness, Kim Petras, Questlove and more.

The costumes were certainly impressive ... and cap off a packed weekend that was full of famous monsters and ghouls who did back-to-back-to-back nights in parties -- with Heidi's obviously being the last stop.