i have seen this happen with much larger gars in public aquariums; where it appears that their faces are being eaten away by some sort of disease, primarily in the grooves of the bony plates.
first thing i would suggest is doing a water change, go with as much as you can up to 50%. add some salt to the tank and increase the temperature to 82F. i would keep up the frequent water changes for the rest of the week, perhaps every other day.
i am not sure what causes or cures this, but it seems to be almost symmetrical on the face of the gar, and if its happening to both it could very well be contagious.
have you added anything different to the tank recently? have they been acting any differently? basically consider anything unusual or different that has gone on in the tank. either way, the water change/salt/increase temp procedure is a good start as you don't want to medicate gars unless you absolutely have to.
if you have the tank space, you may want to set up two quarantine tanks, or at least remove the worse one from the main tank and carry out the treatments there. please keep us posted and good luck with your tropicals--
--solomon