The turtle is one of the few creatures of light which don't recharge you when you're close to them (the only other ones I can think of are the birds from Prairie Peaks*) but it still produces light in some way (the wax). I wonder how it relates taxonomically to the other creatures of light, who recharge your light when you're near them (butterflies, manta, birds). There must be a phylum or some taxonomical group for creatures of light which don't emit light, or/and the ones who emit a very small amount of light (the manatees)
Or perhaps the turtle is in some taxonomical group of creature of darkness, perhaps it feeds on light and the "wax" is its feeces. Maybe this means that the turtle belongs in the taxonomical group that ties between creatures of light and creatures of darkness?
*The birds of prairie peaks are the only birds in the entirety of sky cotl which don't recharge you. I lean to think that maybe it's something that the development team forgot about, rather than an intentional choice, and if it is an intentional choice I believe it's to add to the ways in which Prairie Peaks is supposed to evoke the moth experience of having few places to recharge your light when you run out. I don't think it says anything about the birds of sky that would be consistent with the rest of the birds we've seen.
Maybe this is somehow a type of bird which doesn't recharge you with light/not a light creature? But there are at least two kinds of birds there so it feels very unlikely. I don't remember for certain if the Little Prince's birds recharged your light, I think they did, and if they did, that would make Prairie Peaks' birds nonsensical and inconsistent with the rest of the birds of sky, because Little Prince's birds are supposed to be normal, earthly, migratory birds, which means that in sky, birds should be creatures of light regardless of their types and origins