Imagery is descriptive language, which appeals to the five senses of sight, sound, smell, taste, and touch. Diction is how a poet chooses words to create a distinct style. Mood is the emotional effect of the poem.
First of all, consider the appeals to senses. He describes the fog visually, with “little cat feet” and “silent haunches” so that we can picture the fog, metaphorically, as cat. When I read this poem, I can feel the fog too. The metaphor of the cat creates an image in my head.
Consider the words Sandburg chooses to create this image. The cat is described as “little” and silent. It sits “looking” and “moves on” to somewhere else. One of the most significant word choices for Sandburg’s diction is “cat” because cat becomes the metaphor to create the image throughout the poem, and the cat creates the mood. Notice there are never any real negative words associated with the cat or the fog.
It does partially depend on how you feel about cats, but I get kind of a warm and fuzzy feeling with this poem. If you don’t like cats, or even if you do, there is definitely a sense of mystery and aloofness. I don’t think the mood is scary, because Sandburg uses the word “little” to disarm any fears you have of cats. So the mood is one of mystery and peacefulness.