How can I get the floating treasure ball on Atlas' hand?
You will need to revisit the area after you get to a certain point in Episode 3 Part 2 (Sunleth Waterscape) where you will be given the Moogle Throw ability, this part will also teach you how to use it.
There is plenty of other treasure that can be obtained the same manner, but you will need to progress the game to at least this point first.
This will be available later on with the moogle throw ability.