My son has a PWS on his face as well and has had treatment with the VBeam. My understanding is that the grey spots appear when the laser settings are more intense. These spots seem to be more vulnerable to dryness and possibly crusting. When this happens, we use extra Aquaphor and are very careful to avoid any contact. So far, all his grey spots have healed just as well as the rest of his face and sometimes even look better.
Good luck with your treatment. Hope this helps.