I believe it is Boytritis which is caused by damp cool conditions. I found a link here:
Botrytis blight factsheet
with orchids it is not as dire, but you will want to let the plant dry out a bit, and put it somewhere that it is warmer. It will not come off the current blooms, but with good culture it should hopefully not come out on any buds not opened yet. Keeping it warmer and a little dryer, the next time it blooms it should be fine. Cinnamon spray might help. If you just purchased it, it is likely the conditions at the place where you got it was the problem. If this is a new plant you might want to check the roots to make sure the mix they are in is okay and they are not too wet.