AI or humanoid robots in that era/universe of the show could capture and preserve images through their lens (referring to the 'fluorite eyes') as memories. So it's not necessarily the song is about her eyes, but just as you might have seen in the intro OP (opening theme song), it is all that she has seen in her lifetime. It is essentially her story, her life, all of her experiences that allowed her to be "Vivy" (or her true self) in those final moments of the final episode. The 'song' part of "Fluorite Eye's Song" is the song sung by Vivy in the final episode, where she sings her 'story' - you could say that it was intended to show how Vivy is more than just an AI, but more closer to a human - an AI who sings her life story, with each memory, each emotion entangled into a single song.