Nothing's going to work. You're welcome.
If it were not cotton, or printed, or an octopus, maybe. Or wasn't a piece of cotton fabric glued to a cotton backing with adhesive that will wick yellow up through the face.
Clean it first and see what happens. The yellow adhesive and print will make the rest of the discussion moot.
Or, wow the poop out of the guy by dropping $42 bucks on a new one on Etsy.
blue octopus rug
PS: Just to not be 100% negative... Some have had good fortune using CLO2 on moldy synthetic outdoor cushions, seems I remember some discussion here about that product. Give it a try if the print doesn't bleed.
PPS: I know the link is to a smaller rug. The big one should be $84.