This is a common symptom of several fish diseases, most notably marine ich (cryptocaryon irritans), also known as saltwater ich, or velvet disease (amyloodinium ocellatum).
Marine ich is a parasitic protozoan that causes small, white spots on the fins, body, and gills of the fish. These parasites embed themselves in the fish's skin and cause irritation.
Velvet disease is another parasitic disease that causes a "dusting" of fine, whitish spots on the fish, which may look like a powdery coating. It's highly contagious and can be more rapid and lethal than ich.
To determine the correct course of action, it would be important to have a closer look at the fish and potentially consult a veterinarian specializing in aquatic animals or an experienced aquarist. The user should be advised to research and consider safe treatment options for their specific tank setup and fish species.