The R. or short - Rust - is a kind of moss that settles on metal. It is miniscule, but strong over time. Rust eats through the strongest metals, even those marked down for "rust free". It will not go for living organisms, but if you rub over the affected metal with bare skin, you can get some nasty scratches and small bites.
How to fight it
Since many of our monsterhunter tools are made from metal, the R. can be quite dangerous, because it tarnishes our trustworthy weapons and defenses. To prevent it getting a grip, you have to be careful with your metal and keep everything really dry (The R. loves moist surroundings like all mosses). If once infected, there is nothing for it but cutting the infected areas and shaving the R. off. I have to admit though.... its quite pretty...gab