How to prevent those telltale spots.
Graters are typically made of stainless steel, so they're not supposed to rust, but that telltale brown can start to creep up all the same as you use, wash, and use your grater again. The culprit is usually a dishwashing liquid or a cleanser that's high in chlorides, which may cause rust marks if a residue is left on the steel.
To thwart (or at least forestall) rust, be sure to rinse your grater completely―whether this means passing it under the tap an extra time or simply having faith in your dishwasher's rinse cycle. Water that's especially hard (high in mineral content) can stain if droplets are allowed to dry on the grater's surface, so thoroughly pat yours down instead of leaving it to drip-dry.