The fog comes on little cat feet. (1-2)
It's pretty amazing to suddenly see a fog with little cat feet. Immediately we understand that the speaker is so inspired by what he's seeing that he needs to come up with the perfect metaphor to get his vision across. And it works mighty well, doesn't it?
It sits looking over harbor and city (3-4)
The fog is hovering over us, reminding us that it's bigger than us and can therefore do what it wants. Since it's "looking over" us, we also know we're a bit vulnerable to it which can leave us feeling in awe and perhaps even inspired.
and then moves on. (6)
It comes just as easily as it goes. One minute it's there and we're wondering what will happen next, and then it's gone, like most amazing things.