In terms of literal description, the whiteness refers to the foaming of the sea water as it washes over the rock.
The rock is first described when Ralph crosses the neck to the Castle Rock:
Now he saw the landsman's view of the swell and it seemed like the breathing of some stupendous creature. Slowly the waters sank among the rocks, revealing pink tables of granite, strange growths of coral, polyp and weed. Down, down, the waters went, whispering like the wind among the heads of the forest. There was one flat rock there, spread like a table, where the waters sucking down on the four weedy sides made them seem like cliffs. Then the sleeping leviathan breathed out - the waters rose, the weed streamed, and the water boiled over the table rock with a roar. There was no sense of the passage of waves, only this minute-long fall and rise and fall.
It is described again, being the rock Piggy lands on when he is knocked to his death:
Piggy fell forty feet and landed on his back across that square, red rock in the sea. His head opened and stuff came out and turned red. Piggy's arms and legs twitched a bit, like a pig's after it has been killed. Then the sea breathed again in a long, slow sigh, the water boiled white and pink over the rock; and when it went, sucking back again, the body of Piggy was gone.
immediately before the section you are asking about it is described again:
Every minute the water breathed around the rock and flowered into a
field of whiteness.
When Ralph yearns for a bed and sheets, he is yearning for the clean, safe harbour of home. The imagery of 'sheets' relates to whiteness as both the default colour of sheets and the remembered, idealised, clean, simplicity of 'civilisation' in contrast to the dark, chaotic complexities of life on the island, but the only whiteness where Ralph is now is the whiteness of the leviathan’s breathing. Remember also that when Piggy is hit by the rock the conch, which represented civilisation and civil discourse explodes into 'a thousand white fragments'
Now Ralph likens that whiteness where Piggy died not just to milk, but to spilt milk, evoking the proverb
There is no point in crying over spilt milk
to express regret about something that has already happened or cannot be changed
Piggy's brains can no more be put back in his head than spilled milk can be put back in a jug, its done, crying won't change it.
Piggy was everywhere, was on this neck, was become terrible in darkness and death.
I think this is to be read in terms of the previous descriptions of the rock; the rock and the sea around it have been described in terms of a living creature, a leviathan. Leviathan is defined in the OED as
The name of some aquatic animal (real or imaginary) of enormous size, frequently mentioned in Hebrew poetry.
but also an obsolete definition is given
The great enemy of God, Satan.
To my reading, in Ralph's mind, in Piggy's death on the altar table of the rock, his friend was not only consumed by the breath of the leviathan but has in essence become the leviathan.
In the same way that Simon understood that the Beast was in each of them; and that for the tribe, even as they collectively became the Beast, Ralph became the external representation of it, to be hunted, Piggy has himself become a part of the greater Beast, beyond the puny beast that the imagination of schoolboys brought forth.
Just after the section you quote we read:
Here - and his hands touched grass - was a place to be in for the night, not far from the tribe, so that if the horrors of the supernatural emerged one could at least mix with humans for the time being, even if it meant...
I interpret this to mean that the 'darkness and death' in which Piggy has become terrible is part of the supernatural world and as such to be feared even more than the beast within mortals.
A late edit: I have just come across my school’s magazine from 1981. I had no recollection of this but apparently we had a question when reading ‘Lord of the Flies’ and wrote to Wm Golding seeking enlightenment. History does not relate what our question was, but his reply is as follows:
Thank you for your letter.
I’ve moved from the position I once held: which was that an author knows all about his own books. So I’ll have to say now that your/their guess is as good as mine.
But in fact, as I wrote the book over a quarter of a century ago I’ve simply forgotten what I meant anyway!