Mounts allow players to travel around Eorzea faster.
After acquiring a mount, it can be summoned by dragging the icon on the action bar and clicking the icon. Players can acquire their first mount, Company Chocobo, after completing the level 20 main story quest titled Main Scenario Quest A Hero in the Making and joining a Grand Company.
Players must also complete the quest Feature Quest My Little Chocobo given by an NPC located within their Grand Company Building.
Other mounts can be acquired by meeting different requirements. Heavensward, FFXIV’s first expansion, introduced Flying Mounts.
Enemies can still attack mounted players unless they ride a Chocobo Porter.
You need to reach the part in the story where you can get your chocobo mount. It’s at level 20 but you need to do some FATES or Guild Leve quests for the currency to purchase it first.
The Greens are only used for summoning your Chocobo as a COMPANION not as your mount, to fight alongside you, which is a level 30 quest.
Now to answer the OP about the CE Mount, you must have first unlocked the Chocobo mount from the Grand Company quest (to gather 2000 seals) once you have that mount, you can then summon the CE mount.
You only need to do this that one time, as once you unlock it, any other class regardless of level can summon a mount.