If it's really rust, the frets are probably lower quality than most, and the plating is either coming off or wearing off (which would mean you're playing a lot-good for you). If it's not yet out of hand and the frets don't feel rough to the touch, you can clean them up with #0000 steel wool (remove strings first) and keep it from becoming a problem by conditioning your fretboard regularly.
Some frets are nickel silver (read a kind of brass) and can get badly tarnished, especially by some player's sweat and body oils. I first use TarnX to remove all oxidation, polish, and keep clean. 3x or 4x steel wool, polishing paper, or a fret polisher. Can make one from a rubber fingernail polisher.
Could be the environment too. When I lived in China the pollution caused my frets to rust. Mostly on the high up frets that don't get much use. Just some 0000 steel wool and they clean up fine
Some frets are nickel silver (read a kind of brass) and can get badly tarnished, especially by some player's sweat and body oils. I first use TarnX to remove all oxidation, polish, and keep clean. 3x or 4x steel wool, polishing paper, or a fret polisher. Can make one from a rubber fingernail polisher.