Place black-eyed peas in a large container and cover with several inches of cool water; let stand 8 hours to overnight. Drain and rinse.
Pour 6 cups water into a pot; add ham hock. Bring to a boil, reduce heat to medium-low, and simmer until meat is tender, about 1 hour. Transfer ham hock to a plate to cool and reserve 4 cups of the cooking liquid.
Layer soaked peas, andouille sausage, onion, celery, and garlic in the bottom of a slow cooker. Remove meat from ham hock and add meat to slow cooker. Sprinkle bay leaves, cumin, salt, and red pepper flakes over the top. Pour reserved cooking water over peas mixture.
Cook on High until peas are tender, at least 4 hours.