윤슬, yunseul: the glitter of small waves under sun or moonlight.
You will have stood by a river or the sea and watched the surface come apart into light. Korean could have left it at 물비늘, water-scales, which also exists. It made a finer word as well, and kept both.
Yunseul does not happen on still water. The wind has to be up and the surface has to be moving. Only a broken surface can do anything with light; a flat one simply holds the sky and gives you nothing to look at.
The bay at noon, dead calm, is a sheet of grey. An hour later, with the wind up, you cannot look straight at it.