I was helping a friend from Pakistan to improve his English translation for a long, complicated assignment. He had the sentence below. I'd like to know where '镜中花，水中月' comes from. I just translated it as 'illusions'.
如果没有切实可行的行动方案和具体有效的执行措施，
If we do not have a feasible plan of action and specific, effective measures (to achieve our goals),
再美好的目标都将是镜中花，水中月，难以实现。
however admirable our goals, they will remain unachievable illusions.
"水中月" is a buddhism translation. the 梵漢辭典 has entries:
ambu-candra 水中月
ambu-candraka 水月
in 大智度論 釋初品中四緣義第四十九, there're 3 occurrences of "水中月":
[0297a23] . . . 譬如小兒見水中月﹒心生愛著﹒欲取而不能得﹒心懷憂惱
[0297a28] . . . 如水中月﹒雖為虛誑無所有
[0298a05] . . . 譬如小兒見水中月﹒入水求之﹒不得便愁
this version of 大智度論 is translated by Kumārajīva (鳩摩羅什), at about ad 405.
another one, in 大方廣佛華嚴經 卷第二十五, there're 2 occurrences of "水中月":
[0558b01] . . . 九﹒以幻夢影響水中月故﹒一切法平等
[0561a16]. . . 隨順諸法如幻﹒如夢﹒如水中月不二相﹒而起分別種種煩惱及不失業果報
this version of 華嚴經 is translated by buddhabhadra (佛馱跋陀羅), at about ad 412
about 鏡中花, imo, it's an "inspired" term, derived from "ādarśa-maṇḍala-pratibhāsa" (鏡像).
in 大智度論 大智度初品中十喻釋論第十一, there're several occurrences of "鏡中像":
[0101c08] 解了諸法如幻﹒如焰﹒如水中月﹒如虛空﹒如響﹒如犍闥婆城﹒如夢﹒如影﹒如鏡中像﹒如化
[0104c07] . . . 如小兒見鏡中像﹒心樂愛著﹒愛著失已﹒破鏡求索﹒智人笑之
[0104c26] . . . 諸法從因緣生﹒無自性﹒如鏡中像
"鏡中花" as a term, the earliest one i find is in poem "徐州送丘侍御之越" by 皇甫冉, of 唐 dynasty:
時鳥催春色﹒離人惜歳華﹒遠山隨擁傳﹒芳草引還家﹒北固潮當濶﹒西陵路稍斜﹒縱令寒食過﹒猶有鏡中花
my guess (might be wrong, need evidence to prove) is: zen monks in 唐 dynasty used "鏡中花", instead of "鏡中像" (the correct, original term); pair with "水中月", to talk about buddhism.
"花" & "月" (flower & moon) is a perfect match :) better than "像" & "月"
tired now, have fun :)
It comes from an idiom 镜花水月（jìng huā shuǐ yuè）
For example: 《说岳全传》第六一回：“阿弥陀佛，为人在世，原是镜花水月。”《白雪遗音·玉蜻蜓·游庵》：“我和你镜花水月闲消遣，何必名贤胜地逢。”
镜子里的花；水中的月亮。 Flowers in the mirror, moon in the water. I think the translation illusion is proper in your case. You can also translate it as unpractical, unrealistic.
Provenance: 唐·裴休《唐故左街僧录内供奉三教谈论引驾大德安国寺上座赐紫方袍大达法师元秘塔碑铭》：“峥嵘栋梁，一旦而摧。水月镜像，无心去来。”
Flowers in the mirror, Moon in the water.we have seen through surface of water, mirror is the reflection of light and image. Images are not real objects. All stuffs within water and mirror are just projections.
The proper phrase seems to be:
水中月，镜中花
A Chinese-English Dictionary
IDIOM
moon in the water, flowers reflected in the mirror—fantasy; illusion
KEY
PHR
"moon in the water, flowers reflected in the mirror"—fantasy, illusion
现代汉语大词典
比喻无法得到的美好事物。
姚雪垠《李自成》 乱点鸳鸯…使小张鼐心上的人儿登时成了水中月, 镜中花。
