Does the plant show any signs of a problem? Most phrags do well with damp feet, though, there are some that prefer to dry off a bit more. If you have already decided to repot, then that is the best solution. Clean the root system well and put it into a new bark mix. Keep constant air circulation going around your orchids and it should be fine. Most likely it is simply an old mix that has stayed too wet that caused the fungus or perhaps white mold?? Upotting, cleaning, repotting in fresh mix and increased air circulation should help.