The box jellyfish is almost invisible in the water, so dream books read it as an unseen danger, or a hurt that comes from where nothing seemed to be.
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.