The broad-leaved epiphyllum is a beautiful and precious white flower which usually blooms at night, and its blossom only lasts for a brief period. According to a Buddhist legend, the plant blooms only on the birth of divine kings.
This idiom describes thing which disappear shortly after they come into being.

In the Spring and Autumn Period (chūn qiū 春秋时期,770-476 BC), King Jing of the State of Jin (Jìn guó 晋国) fell ill. One night he dreamed that the disease turned into two small figures talking beside him. One said, ‘I’m afraid the doctor will hurt us.’ The other said, ‘Do not worry. We can hide above huang and below gao. Then the doctor will be able to do nothing to us.’ The next day, having examined the king, the doctor said, ‘Your disease is incurable, I am afraid, Your Majesty. It’ above huang and below gao, where no medicine can reach.’
The idiom indicates a hopeless condition.

春秋时期，晋景公有一次得了重病，听说秦国有一个医术很高明的医生，便专程派人去请来。医生还没到。晋景公恍惚中做了个梦。梦见了两个小孩，正悄悄地在他身旁说话。一个说：“那个高明的医生马上就要来了，我看我们这回难逃了，我们躲到什么地方去呢？” 另一个小孩说道：“这没什么可怕的，我们躲到肓的上面，膏的下面，无论他怎样用药，都奈何我们不得。” 不一会儿，秦国的名医到了，立刻被请进了晋景公的卧室替晋景公治病。诊断后，那医生对晋景公说：“这病已没办法治了。疾病在肓之上，膏之下，用灸法攻治不行，扎针又达不到，吃汤药，其效力也达不到。这病是实在没法子治啦。”