15710st Rotor Stain [PLEASE HELP!]

[ywj3110]

Hi friends,

Recently purchased a pre-owned 15710st (2015) which comes has a glass case back. I was just playing around with a phone camera zooming into the case back and spotted a few weird stain (oxidation/tarnish?) marks on the gold plated rotor. There seems to be a tiny white debris next to the letter A which seems to have caused a circular stain around it and the bottom part of the rotor a darker stain throughout. Please see attached photos for reference.

Does anyone know what has caused this? Is this natural thing to happen since it's not pure gold? What can be done to fix this? I'm now completely heartbroken for the fact that I might have just bought a rusty watch...

Any help would be appreciated... Thanks...