Why is my iridescent shark turning white?

Probable reason is your water parameters and other conditions are not right for the fish. These fish (Pangasius hypophthalmus) are usually sold as immature juveniles. They do not usually last long in the aquarium because they usually die of organ failure caused by being kept in the wrong conditions. They are not sharks but large (edible)catfish and under the right conditions will grow to over 36 inches, so they need a minimum sized tank of around 10 feet long with suitable height and depth to match the length. They need a pH of around 7, a dGH of around 10 and a temperature of around 75F. Maybe now you can understand what is wrong with your "Iridescent shark"