okay so because i see some people being just as confused as i am, i will share you guys what i think have happened.
so mitsuki is an immortal jellyfish - which exist in real life - when he disappeared after having sex, he basically reached his sexual maturity and reverted to a sexually immature and colonial stage, after that as we've seen it, he started to grow again.
if you type immortal jellyfish in google the wikipedia page will pop up!! it's pretty interesting actually!!!
I came back after who knows how many years to reread and finish this manga, but I did not remember this being so fucking painful and crying through the whole story