That looks suspiciously like white spot to me. Have you noticed this on any of your other fish?
- Be sure that the temp of your water is same as tank when doing PWC's - if it is considerably colder this will shock the fish and bring about white-spot.
If it is definitely white spot, treat it quicky!
- Online or local LFS will stock treatment for white spot
- You'll need to crank up the temp in your tank to about 30'c - white spot cannot survive in a tank of that temp
- Make sure you provide extra air pumps/oxygen whilst you have the temp up and are treating for white spot.