12:55 PM PT -- 2 Milly tells us he didn't know what Fortnite was until people started contacting him over social media, telling him the game was stealing his dance. The rapper says even his daughter noticed the similarities to his moves.

2 Milly says the dance started showing up in season 5 of Fortnite, and he wasn't alerted to it until a few months after the release. He says Epic took his craft and sold it as their own, and he wants fair compensation.

2 Milly says he's not trying to ruin the game for anyone, but he's calling out Epic for unethical business practices and wants the "Swipe It" dance removed entirely.

The rapper filed legal docs this week against Epic Games -- the company that makes Fortnite Battle Royale -- for allegedly using his popular Milly Rock moves in the game without his permission to capitalize on the popular dance craze ... and make a ton of cash.

According to the docs ... Fortnite players can purchase the dance -- which is called "Swipe It" in the game -- for their characters. Problem is ... 2 Milly claims "Swipe It" is an exact replica of his Milly Rock, but they never asked him to use it or offered to pay him for it.

2 Milly also alleges dance-stealing is a common trend in Fortnite. According to the suit ... the game copied Snoop Dogg's "Drop It Like It's Hot" dance and the famous Carlton dance from 'Fresh Prince of Bel-Air' -- among many others -- without consent or compensation "to exploit African-American talent" for profit.

The rapper is suing Epic for damages and legal fees ... and wants his dance moves removed from the game.

His attorney David L. Hecht of Pierce Bainbridge Beck Price & Hecht LLP tells us his firm is, "proud to stand up for African-American creatives whose expression and likenesses have misappropriated. Epic is enjoying record profits off of downloadable content in Fortnite, yet has failed to pay or even ask permission to use artists’ intellectual property and likenesses over many of its popular emote dances."

We reached out to the Fortnite makers ... no word back so far.