Nope
The riddle:
I am seen,
but I only exist
in dreams of darkness
and beams of light.
In duality I am not
I shine with purple as I am.
Wrong answers:
A moon
The crystal in Crystal world
Terra
Trance
Viral Smoke
Fat Chocobo in Choco's dream
Artemicion
Kuja's clothes
Memoria
Airship holograms
Terra's history hologram
The faces in Oeilvert
The blue / purple orb in Oeilvert
Mental projection of Alexander in Memoria
Memories
Zidane's bedtime story
The migration of souls from Gaia to Terra
Hints:
The answer is not a character.
The answer is not an item.
The answer is not a summon or a summon's attack.
"darkness" is very metaphorical, "light" is not.
The answer is not a living creature.
The answer is an event.