I have a guess as to what "Why-man" is based on the latest two episodes.
It's not human, but a bigger "Medusa" who's been mimicking human sound ("Like a Vocaloid," as Senku once said) and has been shedding tiny versions of itself onto earth. They might have gotten the first voice from Byakuya or from somebody who was in that ship.
This guess is based on the fact that there would've either have to have been people repopulating on the moon for billions of years so they could continue trying to freeze the entire world, and that they'd have to have constant tabs on human civilization in order to know that they have to freeze them again.
That, or there's a rocket ship elsewhere on earth, but they probably would've found evidence of that with the satellite.
I think it just makes a lot more sense for it to be a moonling creature that's trying to sabotage the earth because of like... Pollution or something.
And Senku and Xeno will be like "I knew it!"
I know I could just look it up, but I don't want to. I wanna wait and find out on my own.