The living things are long and complicated, and past lives is as simple as a dustpan, with green mountains and green waters, and people are old and yellow. What does the author want to express?

Past lives is as simple as a dustpan: simply leave now.

Green mountains and green waters still exist: green mountains are not old, and green waters flow forever.

An old man's epiphyllum will change: but an old friend will wither and grow old like an epiphyllum.

The author's whole poem expresses an emotional artistic conception: time flies, beauty is easy to get old, things are wrong, things are wrong, and they are done and cherished.