Either it got bitten by tankmate, or water is not clean. Looks like fungus or viral infection. Do a 40% water change, raise tank temp to 86F-88F (make sure provide a lot of dissolve oxygen), add salt (a handful every 100 gallon of water.) You can use Mardel Maracyn Plus (liquid) to treat the water (following recommend dosage on the back), or Coppesafe (I found Mardel Maracyn Plus works great with arowana, and doesn't cause much stress to the fish.) Do a 30% water change every 3 days, and repeat the treatment at half recommend dosage, until symptom is cleared. If you have a UV sterilizer connected to your system, make sure to unplug it before you add medicine to your tank, and leave it off until you finish treating your fish with medicine. You need to do at least two weekly water change after finish treating your tank with medicine before you can turn the UV back on, since UV can break down the medicine and turns it in to toxic. Once you got rid of the problem, 30%-40% weekly water change is recommended, and wash clean the filter pad each water change. If you use canister filter, dump the water inside each time you wash the filter pad. Don't wash your bio-media. Good luck!
