Hi ya, that looks like fungus but just to be sure maybe you can get a skewer an give em a little poke before you unpot it. The bark (that's what it is right?) may be staying moist in that spot longer. Add a fungicide plus a dash of pesticide to the next watering just incase, if that doesn't help then you'll have to repot and mix in something like lava rock or packing peanuts or hydroton (clay balls) to give it a bit more air flow. Hope that helps.