Marine ich (Cryptocaryon irritans) is a common parasite that shows as tiny white salt-grain spots on a fish, usually triggered by stress or a temperature or parameter swing.
Because part of its life cycle is off the fish, treatment must run in a separate quarantine tank and long enough to catch the free-swimming stage — most reef-tank medications are toxic to corals and inverts. This is general guidance, not a diagnosis; get experienced eyes on a serious case.