Hi,
Based on your description, your Flowerhorn most likely has White Spot Disease (Ich/Ichthyophthirius multifiliis), which is a common parasitic infection. Loss of appetite is a sign that the infection may be advanced, so treatment should be started immediately.
Recommended Treatment:

1. Raise the water temperature gradually to 30°C (86°F) over 24 hours if your aquarium setup allows it.
2. Increase aeration, as warmer water contains less oxygen.
3. Perform a 30–40% water change before adding any medication.
4. Use a reputed anti-Ich medication (containing Malachite Green, Formalin, or another Ich-specific treatment) exactly as directed on the label.
5. Remove activated carbon from the filter during medication, as it can absorb the medicine.
6. Continue treatment for 3–5 days after the white spots disappear to ensure the parasite is completely eliminated.
7. Do not overfeed. If the fish is not eating, focus on maintaining excellent water quality until it starts recovering.

Please share the following details so I can guide you more accurately:

• A clear photo of the fish.
• Tank size (in liters/gallons).
• Water temperature.
• Are there any other fish in the aquarium, and are they showing white spots too?
• How many days have the white spots been present?

Early treatment usually results in a good recovery, but if the fish is lying at the bottom, breathing heavily, or the white spots have covered most of the body, immediate treatment is essential.
Regards,
Yashwant Naik