The undefeated 42-year-old was signing autographs at the Art of Music shop inside the MGM Grand in Vegas over the weekend ... when a fan asked Floyd to sign a ticket from the 2015 Mayweather vs. Pacquiao fight.
Get this ... fans were NOT allowed to carry cell phones while interacting with Floyd, but when Mayweather saw the item, he told the fan to go out and grab his phone so he could record "something special."
When the guy returned, Floyd put on a show -- saying, "So, what I'm about to do right now is sign 'Victim 48,'" Floyd said ... a reference to Manny being the 48th fighter Floyd defeated.
"So, what I'm gonna go for you ... I gotta cross him out!"
That's when Floyd grabbed a pen and scratched a big ol' "X" over Manny's face!