You should listen to me. No, seriously, listen to me.
Join Date: Oct 2008
Location: Santa Barbara
Motorcycle: Blue 2006 GSX-R1000
Re: Oxidation on gas cap, possibly inside tank too!
That's "rust" on zinc die casting. The bike has likely sat for some time, probably with a near empty tank, and breathing due to temperature changes has brought moisture laden air inside the tank. Humid environments like in Florida help things along. You need to get that out of there because it's next stop is your fuel filter, which is expensive to replace. There are some iriditing (chromate conversion)-type treatments that will help hold it at bay and aren't that expensive.
Definitely check out the inside of the tank.