Slag is the waste left after metal is purified, so dream books read it as what remains after effort — the unwanted part that is cast aside so the valuable part can shine.
Has this come up in your dreams before? What was going on in your life then?
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.