My flowerhorn has white spots on its head. How do I treat it?

The addition of live prey may have introduced parasites into your aquarium. It is also possible it was pre existing and an unknown stressor may have triggered active infection.
Regardless, now you have to treat. Check water quality first. Poor water quality is a major cause of illness and it can also complicate any condition. Test a sample at home with a kit or submit a sample for testing. Most pet stores or aquarists will do this for free or for a nominal fee. Ammonia and nitrites should be 0 ppm (parts per million). Nitrates should be 40 ppm or less but for HITH, try for 10ppm or less. Perform partial water changes using aged or conditioned water that is temperature and pH matched to prevent shock and further stress until water parameters are acceptable.
Metronidazole is the treatment of choice for HITH. It will kill Hexamita which is the parasite associated with HITH. You can purchase metronidazole for tanks online or in the aquarium section of most pet stores. Follow label directions carefully. Remove the charcoal from the filter during treatment. A broad spectrum antibiotic should also be used to prevent secondary infection. If you have other fish in the tank, monitor them for similar symptoms.
A proper diet is also critical. A vitamin and mineral deficiency may predispose a fish to this condition. Use a fortified pelleted diet. You can soak food in garlic juice (or GarlicGuard by SeaChem) to boost the immune system and stimulate the appetite.