Hmm... I go with the protodermis whirlpool theory.
We have so much teasers about protodermis and the Toa Nuva that we can practically conclude that protodermis helped the Toa Nuva evolution.
You can also see this from an artistic point of sense. You can see that Lewa's axe is warped, or distorted. That probably means that light is being refracted in crazy ways, and something needs to refract it. The 'lens' may just be protodermis in a thinner, clearer state OR Lewa's axe isn't too far from the inner wall of the whirlpool so we can see it.
But what if there's another form
The Onu-Korans mine protodermis, and they talk about it being in layers
. Liquid doesn't have layers, since it has no absolute shape. That means that other than the silvery liquid, protodermis can be hardened. The liquid, as Torhu said, might be
protodermis' purest state.
But what if there's a gas form of protodermis? Liquids evaporate, right? But I don't know how it can get so hot in the nest, though. It would have to be boiling in there for protodermis to turn into vapor at such a rapid rate that it turns into a cyclone. Maybe the refraction is caused by some liquid caught in the cyclone. Raindrops, no matter how few, still reflect light.
Or maybe Lewa is simply being warped. Maybe they don't come out of the protodermis AS the Toa Nuva, and develop later on inside a mini-cyclone. You saw the picture of all the Toa Nuva standing in a pool of protodermis. Part of their feet are submerged, meaning that the pool is shallow. They had walked out already, and evolved later on.
BOY I wish I could talk to the real Toa Nuva right now...