What does Bai Juyi mean by "flowers are not flowers, fog is not fog"?

The whole poem is full of metaphors, flowers, fog, spring dreams, clouds and beautiful images of nature. From form to spirit, it seems to be right at first guess, but it is denied one by one. There are hidden things that are beautiful. It is obscure, short-lived, perishable and uncontrollable. Some people say it's singing official songs and geisha. Ge Ji looks like a flower, but it's not a real flower. Geisha, unlike their wives, can only have fun at night. How long will it be like a spring dream? Leave like a sigh in the clouds. The last sentence will be short, but not too long. Because of the clever use of language and writing, the love between men and women is implicit and poetic. Bai Juyi's poems always expose corruption, hate officials, write about people's livelihood and express their sufferings. This poem is completely out of his style. Some people say that Bai Juyi wrote about acacia. Expressing that kind of admiration and passionate love for beautiful women, sleepless all night, unpredictable, unable to dispel, beautiful but far away as clouds. This uncontrollable feeling of love, which Bai Juyi can't say clearly, can only be expressed implicitly.