Here is the best option for treating a Saltwater Angelfish with white growths. Treat with Marine Melafix. If it does not clear up, but the Angelfish is eating well it may be alright. Saltwater Angelfish are prone to external tumors that are generally not life threatening. If they continue to get larger, but do not multiply you can clip them out of the saltwater angelfish with nail clippers. After clipping, treat with an antibiotic like Fish Mox. You may need to get this online.
Concentrate on good nutrition. Feed a good Frozen Angel specific food like Hakari Mega Marine Angel cubes. Saltwater Angelfish need sponge in their diet and you won’t get this in most fish foods.