月とスッポン (tsuki to suppon) (also “月と鼈” or “月とすっぽん”) literally means “the moon and a (Chinese) soft-shell turtle”. Figuratively, it describes two things that are utterly different from each other. While it could be translated as “(be different like) chalk and cheese” or “apples and oranges” in English, the Japanese idiom slightly differs from them in that it usually suggests that one is much better than the other, with the moon indicating the better.
This idiom is often used when you compare two things/people and emphasise that one is undoubtedly superior to the other. For instance, you may compare the governments of two countries and say they are 月とすっぽん, highlighting how good/bad one of the governments is compared to the other.
It is widely believed that this idiom alludes to the fact that the moon and the shell of a turtle look similar in terms of shape (i.e. both are round) but very different in terms of beauty (i.e. the moon looks much more beautiful than a turtle).
スッポン (Suppon) in Japan
In Japan, スッポン (a Chinese soft-shell turtle) is regarded as one of the best food ingredients for fueling (men’s) sexual energy and stamina as well as for improving one’s health and beauty — although it is not commonly consumed in Japan. Famous Japanese food using スッポン includes “Suppon hot pot”, “Suppon soup”, and even “Suppon blood mixed with wine/sake”, which you may want to try in Japan to get your blood pumping.
His acting and hers were exactly “the moon and suppon” (i.e. His acting was way much better than hers)
五十歩百歩 (gojuppo hyappo)