Damp creeps in slowly and spoils from within, so it is read as a lingering low mood or a neglected issue. Damp spreading over the walls stands for something left unattended for too long.
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.