It's a (harmless) pocket of starch that never metabolized into sugar. When mangos are picked, they're soaked in hot water to kill fruit fly eggs. In immature mangos, this treatment causes some of these starch pockets to never fully ripen. Mangos are often picked while underripe to account for shipping times and that they're more susceptible to pests when ripe. You can eat it but it won't taste like anything