Rebbe Yid: This is a troll thread. My first statement was clearly a joke.
The Mishnah is clearly referring to Atlas, who is always holding a ball. But the ball he is holding is the Aristotelean “Heavenly sphere” the makes up the entirety of the cosmos, not just the planet Earth. That is what is referred to in the Mishnah and the first statement in the Yerushalmi. The second statement, about Alexander, is referring to Earth, with the Yam as a plate holding this planet.