Gills could be infected, showing signs of Marine velvet?
Marine Velvet (Amyloodinium ocellatum)
Symptoms: Respiratory distress (fast breathing - gills opening more than 80 times per minute); White, yellow to light brown, or grey "dusty" appearance on body, Loss of appetite, Rubbing or scratching against decor or substrate.
Marine velvet is one of the most common maladies experienced in the marine aquarium, with the other being Marine Ich. It is found in all the oceans of the world and often infects wild and newly caught marine fish. It is a fast moving disease that can cause mass casualties. Primarily it infects the gills of fish but can attach itself to the body as well, burrowing deep into the skin's subcutaneous layer. Deaths are generally a result of interference to the respiratory system. This disease is highly contagious and fatal.
Chemical treatments for this disease include using copper. Follow the instructions provided by the manufacturer. Natural methods include hyposalinity, a quarantine tank with a low salinity. A danger with with using low salinity is in re-acclimating the fish to a higher salinity. You must be able to accurately measure the salinity and must increase it very slowly.