Not to completely dork out, but:
"Most important for adventurers to note, however, are the pair of thin antennae that rise up from beneath the beast's eyes. These antennae allow the rust monster to corrode and draw sustenance from metallic objects."
I believe the rust is the byproduct of the Rust Monster's consumption. It does not eat the rust per-say.
Always keep a good set of leather armor on hand. And always let your mom know you're having friends over late for your 38th birthday.
-=- You eat one lousy foot and they call you a cannibal. What a world. -=-