Doesn't look like any parasitic infections that I know of.
Doesn't look like any viral infections that I know of.
Doesn't look like any common bacterial infection + progression I know of but could be a slow secondary infection due to stress or something else in the tank. Has there been any changes to the decor or equipment within the past 3 weeks? Also what's the water parameters (ammonia, nitrite, nitrate, and pH)? Is there activated carbon in the filter, I recall hearing arowana have a high tendency to develop issues when there is although I've never researched deep into this topic since i've never used activated carbon in my arowana tanks.
TBH it looks like scrap marks or very, very light heater burns. I would stick with water changes and no medication for now and document changes. Also keep a look out if you arowana is jumping into the lid or not. Finally, if you're not doing this already, make sure to clean up your gravel as detrimental bacteria like columnaris are essentially always present in the water column and can outbreak quickly if there is a lot of left over food in your substrate.
