When did the Japanese moustache flourish? Is there any special meaning?

That kind of beard is called sanitary beard, which was popular during World War II. Hitler kept that kind of beard to facilitate cleaning.

After Meiji Restoration, Japan began to study western culture in an all-round way, ordered civil servants to wear kimonos and keep traditional hairstyles, and demanded that clothing culture should also be fully westernized. The main country in Japan to learn western culture is Germany, which has many similarities with Japan at that time. It is a dual constitutional monarchy, which has just eliminated the feudal separatist forces. Therefore, Japan pays special attention to the development model of Germany and has sent delegations to study in Germany many times. At that time, Japan learned everything from Germany, copied German laws, learned German political and military systems, and even learned German habits of studying in Germany. Therefore, the Japanese sanitary beard is related to Hitler's sanitary beard.