Migrants, Immigrants, Emigrants, Refugees, Exiles, Expatriates and Others.

IMIN is the Japanese word written with two Chinese characters 移民. 民 indicates people and 移 means: move, change, transfer, shift, devolve, switch, or displace, remove, turn over or shuffle.
移民 IMIN more often refers to emigrants who moved to South or North America and Hawaii before WW2 rather than immigrants. Or as a direct translation from the English word "immigrant" referring to immigrants and the immigrant problem abroad.
For example Korean residents are the largest ethnic minority in Japan. Some of them are fourth generation. They are not mentioned as "IMIN" but are specifically called Korean or Chosun residents in Japan. 在日韓国人、在日朝鮮人、在日コリアン. Later residents from Korea who came after 1980s have sometimes been called  "New comer(s)" ニューカマー.

During the 1980's to 90's many illegal foreign workers came to Japan, from Korea, China, Taiwan or Pakistan, Iran, Bangladesh, also from South America, overstaying their visas. Even genuine economic refugees are never referred to as 移民 IMIN or immigrants, but are rather just called overstayers in Japanese.
出稼ぎ "Dekasegui" was originally used for off-season jobs for farmers from poor rural areas in Japan, and literally means going out to make money in Japanese. Recently in Latin America the word has been adopted into Portuguese and Spanish and denotes ethnic Japanese living in South America who have migrated back to Japan. In 1990 the Japanese government started to issue special long term resident permits to Nikkei(s) 日系: descendants of Japanese emigrants. This group is not referred to as 移民 IMIN either, they are mostly called 日系 Nikkei Brazilians or Nikkei Peruvians.

Legal or illegal, foreign workers are called foreign workers in Japanese, 外国人労働者. 外国人 (Gaikoku-Jin) means "Foreigner" or "Alien" and denotes a person 人 from a foreign country 外国. 外人 (Gai-jin) is the same meaning denotes a person from outside 外. It is not slang, just a shorter and casual version of the same word. Most Japanese don't know that in recent years foreigners in Japan have taken it as a discriminatory term.
移民 IMIN has never been a popular term to refer to foreign residents in Japan. For the moment IMIN still remain invisible in Japanese society.