I'm not 100% sure but I'm leaning towards kno4te's diagnosis: infection, possibly lymphocystis related -- like my Kamfa probably had (and kno4te helped diagnose) -- that red pimple/abscess looks quite similar to the one my fish had, which grew quite quickly and then literally exploded.
Luckily it was on his side near a pectoral fin -- if on his head I don't think he would have survived. You can read about and see photos here:
https://www.monsterfishkeepers.com/forums/threads/kinda-gruesome-wound-recovery.713848/
If it is this, then you need to act fast -- first a 30% water change + salt added, then ideally within a couple of hours, with strong anti-bacterial meds. Again I'm not sure your fish has the same problem, just saying I highly suspect it's the case.
Btw, if you can't easily get the meds kno4te suggests, and esp. if you live in Asia, I would recommend to try and get some of Ocean Free's Flowerhorn line of meds:
Can't guarantee same results with your fish, but the orange Ulcer & Internal Bacteria Away + green Fungus External Bacteria away is what saved my fish. For reference I did not give them at the same time: first I gave the internal one, then after several days did a water change and then added the external one. I did this series twice (again with water changes).
Good luck and let us know how it goes.