That is either a fungus or an algae, possibly caused by overwatering or keeping too damp. I would pull the plant from its pot, soak the pot in hot water then some copper oxychloride, then soak the roots in some copper oxychloride too. Remember for the soaking part, mix about 1 part copper (liquid or powdered) to 3 parts water. After you have repotted with fresh bark, carry on with a regime over 6 weeks of spraying weekly with a quarter strength copper oxychloride solution. Then after this, start fertilizing weekly, weakly with a proper orchid fertilizer. Never mix copper oxychloride and orchid fertilizers, i did that once and it killed the plant.
I wouldnt take any risks, and i would check to see if there is any root damage down below.
Good luck! Jordan