Those are Neobenedenia monogeneans (flukes). They show up well on a dark colored fish. Treat with Prazipro (more info below).
Also see here for more detailed info on flukes:
https://www.reef2reef.com/threads/flukes-–-general-guidelines.224423/
Prazipro (praziquantel): Treats flukes, black ich, and some internal parasites (worms).
How To Treat - In either a quarantine or display tank, dose
Prazipro at the rate of 1 teaspoon per 20 gallons of water.
DO NOT OVERDOSE (especially with wrasses), try not to mix with other medications (for various reasons), and provide additional gas exchange while treating with Prazi. Wait 5-7 days, do a 20-25% water change and then repeat dosage. The reason for the second dose is to eradicate the “next generation” of worms before they can lay eggs of their own. Because while Prazi does kill worms, it doesn’t eliminate any eggs they might leave behind.
Prazipro is generally considered reef safe, although it may kill any tube worms/feathers dusters you have. It may also eradicate bristle worms. If you have mass quantities of these, the resulting die-off can lead to an ammonia spike. After treatment is done, activated carbon may be used to remove any residuals (if you need to use a different medication next). If using a protein skimmer post-treatment, be advised that it will “over skim” for at least a couple of weeks.
Pros - Reef safe, effective dewormer that is relatively gentle on most fish.
Cons/Side Effects - Mild appetite suppression, moderate oxygen depletion, wrasses are sensitive to overdosing.