Why is Moby-Dick white?

I believe he was just born as an albino whale. Look back at chapter 42: “The Whiteness of The Whale” for more info.

Two actual events inspired Melville’s tale. One was the sinking of the Nantucket whaling ship Essex, which foundered in 1820 after it was attacked by an 80-ton sperm whale 2,000 miles (3,700 km) from the western coast of South America. First mate Owen Chase, one of eight survivors, recorded the events in his 1821 Narrative of the Most Extraordinary and Distressing Shipwreck of the Whale-Ship Essex. Already out-of-print, the book was rare even at the time.[2] Knowing that his son-in law was looking for it, Lemuel Shaw managed to find a copy and buy it for him. When Melville received it, he fell to it almost immediately, heavily annotating it.The other event was the alleged killing in the 1830s of the albino sperm whale Mocha Dick, a name derived from his home in the waters off the Chilean island of Mocha. Riddled with dozens of harpoons from his numerous escapes from whalers, Mocha Dick often attacked ships with premeditated ferocity. One of his battles with a whaler served as subject for an article by Jeremiah N. Reynolds[4] in The Knickerbocker, New York Monthly Magazine, which Melville would likely have come across through his literary connections or during his time in New York City.[

Why is Moby dick a sperm whal that ate Sea men

For the purpose of the story, and the symbolism, what I was taught was that it symbolises purity, innocence, nature, that sort of stuff.

Because most sperm whales are white (or gray).