This is pretty interesting, i wonder what is Milotic thinking
I'm not confident enough to leave my own translation but basically Lusamine is praising it for its beauty and color, and saying it's her most beautiful child and thus it deserves all her attention. I'm sure it's remembering when Lusamine loved it simply for what it was rather than its beauty.
What is ugly is merely meaningless.Because of your beauty, you are more worthy of me than anyone else.Ah, you beauty. The elegant fins and rainbow scales.So remain beautiful, for my sake.