Its called fret tarnishing, and happens due to the metal's being exposed to environmental stuff. Sounds like you may be in a humid area, combined with heat and or sweat which is common for guitars to go through. You need to keep your guitar wiped down after you play it. The fret tarnishing is cleaned with steel wool among a few other things, its good to give a good polish on a regular basisDURBANS said:Oh and that thing on the frets it's kinda like rust, but it's yellow and looks like it's part of the frets. Dunno how to take it off either, it's not rust.