Colby just went off about Nick during an interview with Helen Yee -- claiming he had signed a contract to fight Nick at Madison Square Garden in 2018 ... but Nick wouldn't put pen to paper.

"Nick was on board, he gave his verbal to the UFC that he wanted to fight me," Covington said ... "but when they sent him the contract, he never signed the contract.

He added, "We find out that he's a Stockton soy boy. He's scared. He has Stockton slaps but I got MAGA bombs and he ain't got sh*t on MAGA bombs."

So, when we saw Nick leaving Catch in L.A. on Monday, he disputed Colby's version of the story -- saying, "I'm not gonna sit here and call the man a liar, I never heard about it. I just found out."

But, Nick isn't a guy to dive into the details, he simply made a street offer to Colby instead.

"If you wanna fight man, you know where to find me. I'm not a hard guy to find ... step in my motherf*cking range right here."

He added, "Come at me, bro!!!!"

We also asked Nick about Georges St-Pierre -- and Diaz told us that there's no bad blood, he actually likes GSP ... he just wants the guy to move on with his life.