White-ish areas like that should be viewed with great suspicion. It can be a sign of a bacterial infection. Two ways to deal with this situation. One is to support the fish's immune system as much as possible. Feeding a wide variety of high quality fresh/frozen foods + vitamins, and antibiotics as needed in either a bath or QT.
Mixing 6-8 assorted frozen fish cubes, placing them into the fridge to "slump" and then mixing them. Strain off the excess liquid as it will spike your nitrates and phosphates. Add to the mix vitamins like Selcon, Zoecon, or preferred Beta Glucan (see link below on the benefits of BG) + Focus to bind and a bit of water to get the consistency you like to work with. All the fish in the tank will benefit from this mix.
For antibiotics, can treat in a bath or QT. Here's a thread by Humblefish that lists many available meds and treatment methods.
Best of luck with your beautiful fish; I have its twin in my tank
By Christine Williams Inevitably as reefkeepers we do things that stress our fish and invertebrates: bringing new animals in, moving them from tank to tank,
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Antibiotic treatment What It Treats – Bacterial infections, which are oftentimes “secondary” to preexisting parasitic (and worm) infestations. How To Treat – Follow the directions on the label of whatever product you are using. Always use a broad spectrum antibiotic medication i.e. one that...
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