Originally Posted by Aluyasha
Could be injuries from rubbing his face on the divider while flaring at the other male.
Whatever it may be, you might want to QT him. Considering that, if it is fungus, it can infect your other Betta.
~poke~ if one fish in the tank has a fungus, the whole tank has it. I had a pair of new Deep Blue 2gal that had fungus from original manufacture once...
It looks to me like something that's already healed, I've also seen those appear. How's the surface agitation of the water on both sides?