This is clearly an allegory about foreigner immigration to Japan. "alien" is either referring to Koreans or Americans. Either way it's hypocritical and dumb.
A group of people taking over and colonizing a place.... Treating people as subhuman... Almost reminds me of something... Hmmm...
https://en.m.wikipedia.org/wiki/Comfort_women
https://www.britannica.com/place/Korea/Korea-under-Japanese-rule
Yeah, fuck this manga.