Stain on paint after using octane booster

[SSbaby]

Although I poured water over the fuel filler cap, after adding some octane booster, I noticed a stain on the rear bumper the next day, probably from the remaining residue around the cap. I've tried Meguiar's paint cleaner, eucalyptus oil... no luck! It looks like it's eaten into the paintwork.

Is there a product out there that will get it off? :(