I have this idea for a world orbiting a type O star, a particularly active kind of star producing intense radiation. As a form of adaptation, the microbial life has become bright white so to reflect the radiation. This makes the planet look like it's covered in ice when it's actually pretty lukewarm. They've formed a thick film over most of the whole planet beneath which most all of the other life lives to feed off of it. Some of the more advanced life can come out on land for a little while, but only at night to avoid the radiation. Does this ecosystem work? Would white autotrophs even be feasible?
Each plankton organism carries a crystal.
Just as earthly plankton might grow a shell, your world's plankton will grow a crystal. The crystal is precipitated out from the seawater and used as a bulk shield by the organism, which keeps its DNA and most of its mass on the underside. The crystal absorbs most of the radiant energy, and what gets thru and is scattered / refracted back is fine for the organism. Crystal composition varies from organism to organisms - sodium / magnesium chloride is most common but in polar regions there are organisms which purify water and carry a water ice crystal. Some organisms carry a crystal of insoluble white calcium salt - which is converging on a shell or test.
The crystals of salt or ice generally look white. Thus your white plankton sea.
After a storm, great rafts of plankton wash up. On rotting, piles of their crystals remain.
Adapting white would mean they're reflecting visible light spectra. O-type stars emit tremendous amounts of ultraviolet. Sunscreen (which can contain zinc, making them white) is often transparent. My eye glasses block UV, and they're transparent, too. So, the "intense radiation" isn't what's causing the critters to be white. On the other hand, what UV radiation does to skin here on Earth is cause it to produce melanin, turning the skin tan. What this does (among other things) is make the skin opaque to the UV light, reducing the damage it can cause.
However, O-type stars are also very bright and tend to be blue. Let's chase that for a moment. Plants are green because they reflect the blue and green spectra, absorbing instead the red spectra that's more efficient for photosynthesis. But in this case, the plants need to absorb the green and blue spectra because it's mostly what's available (this isn't quite true, but it's true enough for government work). The consequence would be black plants in that they're absorbing everything they can for photosynthesis.
So, in humans the skin darkens in the process of becoming more opaque to block light and in plants they'd turn (IMO) black to absorb as much of the spectra as possible for photosynthesis.
I know your planet doesn't have plant life, but I needed to make the point. On your planet, what we have is the primoridal ooze that's just beginning to create complex life forms. Now that I think about it, you likely do have plants on the surface already. But what would turn them white?
That's actually a problem. Energy needs to come from somewhere or life doesn't find a way. All life begins with the sun, and that means absorbing energy, and that means reflecting nothing or very little.
But let's run for suspension-of-disbelief rather than science-based. What if we assume the creatures in your ooze (not the plants, you're kinda stuck with the plants) developed a skin that needed to act in the opposite way to how skin reacts here on Earth? Rather than blocking or reflecting spectra, we want to absorb it — but not like plants.
What if your creatures developed an epidermal skin layer that worked a bit like glucose-generating solar panels. What you really want is a mirrored surface (aka "white") under the transparent skin so that the skin simply gets baked by that glorious and short-lived star! Now we're cookin' with gas! The star may be hotter than Heather Locklear back in the 80's but your planet is cold! (Oh, all right... lukewarm.) And what this mirrored (white) dermal layer is doing is injecting heat into the body by warming the epidermis and allowing the blood to take the heat inside.
So, lukewarm planet, but critters still need to be warm — especially if the early life forms are cold-blooded!
- Transparent epidermis.
- White dermis.
- Thread-thin blood vessels (so the overall view is white, not pink).
But your plants are still black or very dark red. Sorry.