Do you think that Ruthari's promise "my love will be with you even when the moon is not" potentially might tie into the lullaby Rayla sings to Esmeray, or just part of moonshadow elf culture as a theme?
I think it can be both, tbh. The moon lullaby in S6 ties into a lot of things, such as the season's here motif as a sign of love, as well as the Moon philosophies that tie into the lullaby and Ethari's statement. There's an underpinning of Moonshadow culture / philosophy that things leave but likewise always return, just as the moon moves in and out of its phases. But what do you do when the moon isn't seemingly there at all, when there is 'no light' on a moonless light? Moonshadow elves likewise have a strong emphasis on togetherness and community in their own way, so it would make sense that the Lullaby and Ethari's promise he gives to Runaan and Rayla would tie back to that idea of "My love is with you even when the moon isn't" = "The moon is more than just her light"
So I think Rayla is remembering the lullaby as a way to calm pets (Stella), a reference to Luna Tenebris ("I am the moon, the silver queen"), and something that's deeply meaningful to her precisely because of her bond with Ethari: "But I also know they never really leave us. Luna's love is with you, even when the moon is not." Just because someone - the light, the love - isn't visibly or physically there anymore doesn't mean it's gone, not if you hold onto it and your memories of love. That's not exclusive to Moonshadow culture by any means, but it does seem to apply the most to Rayla's family unit at this point in the series
Rayla's lullaby ("Hold to what you've always known / I am near, my love is here / Though my face cannot be seen / The answer in your heart is clear") also ties in Callum's one truth ceremony. With the starweaver venom and the blindfold, he can't see much of anything ("you need to look inward" / "but I don't see a light, all I see is darkness") but he still finds the answer within himself and within his heart. His truth, his answer, is clear even though he can't see Rayla's face, and even when he couldn't when she was missing: he still always loved her.
