rockm

Probably not fungus, but mineral buildup from watering. See if it's "squishy" o scaly. If it's scaly, it's lime build up from your water and fertilizer supply. It's not really harmful, just a bit unattractive. It's a pain to remove (scrubbing with a toothbrush and diluted vinegar solution) and may not be worth the effort. Scrubbing can do more damage to the bark than the buildup.