Without disputing any of Ofir’s information listed above. I can say that I saw the reed when it was first installed and it was rust free. I never opened it after then. I submit that it may be made of high carbon steel, which will rust and as Melodica me suggests, can be treated with lemon juice to develop a patina, which will resist rust.
As far as the melodica under the sea, I live in a sea port that has very high levels of humidity. That may be the rust origin.
Seal it and perhaps exchange the rusty reed.