Rust is read as neglect and the slow wear of time on skills, ties or plans left unused. Dream books take it as a nudge toward something that needs attention before it weakens further.
When this appeared in your dream, what were you feeling?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.