Mega Hex took the planet, the things on it (the new golden apple), and Mai.
It's easy to say Kouta used up most of his powers dealing with Hex by trying out this and that.

Wait... isn’t the planet Kouta took with Mai another one? Like the one were the new golden fruit is was already inhabited by aliens, didn’t Kouta choose a dead planet and gave live to it?

Unless I mistake something here and Kouta just defended the other planet from Megahex.

MegaHex showed up on Kouta's planet from 'nowhere'.
A new golden fruit is grown every time a chosen one has created a new world, and is ready for the next lucky person.

I thought it is when Helheim invades a new world?
Nvm then, if Kouta brought Invess and Helheim's fruits to the new world is considered as an invasion.

Not exactly. Kouta's world was a black rock when he sent the inves and plants infesting Earth there. There was absolutely no life there whatsoever, so I doubt you could count it as an "invasion".