Other symptoms? Fast breathing, hanging in one spot, diminished apetite, etc?
At least as far as that photo appears to me, hexamita (as in hole in the head) wouldn't be my first thought. Externally, hexamita usually shows primarily or firstly in the sensory pits, whether appearing as small fungus like spots or creating eroded spots -- as in these images: ...
link 1...
link 2
It's possible that's just a scrape that's been attacked by bacteria/fungus-- if it's that simple, water change, salt, melafix, bucket baths with methylene blue, or other similar treatments would normally take care of it. I've also seen something like that appear with certain SA species when ph gets too far out of their range-- they can become susceptible to fungus/bacteria infections.
Everyone has their go to treatments they're going to recommend, but from my experience if it's something more than one those simpler possibilities I'd act fast and wouldn't rely on metro alone, since it has its limitations. See
reference. I'd at least combine it with Kanamycin (page down in reference above) or go to something fairly wide spectrum (for example, Jungle Fungus Guard, or one of the combinations suggested on same page as above-- under
Aquatronics)