Baidu Encyclopedia explains:
1 It is a metaphor for a natural and unrestrained flow of water (mostly referring to writing, singing, poetry, calligraphy, etc.), like floating clouds and flowing water.
2 Describes things that flow and dissipate easily.
3Describes speed and lightness of movement, like walking clouds and flowing water
Source Song. 苏轼《答谢民师书》："Roughly like walking clouds and flowing water, there is no definite quality at the beginning, but always walking in what should be done and stopping in what should not be stopped."
Usage
Juxtaposition: 行云 + 流水; as object, determiner; with positive connotation
Near synonyms
Uninhibited, unrestrained, free-flowing
Antonyms
Pretentious