Can't help in Rethymno but there's an excellent dermatologist in Xania. Georgios Milonakis, Megaro Plaza, (Corner of) Skalidi & Peiraios 1-3 (2nd floor). Turn left out of lift, through doorway to a balcony walkway, 3rd door along (maybe 4th)
Office: 28210 36030 and 28210 44244 Mob: 6944 886 716 Fax: 28210 36031
Gives patients he time they need and English is very competent, just short of fluent.
You'll find the location on Google Maps, here
Peiraios St. is almost opposite Zara. The offices entrance is at the near end of the first building. Parking is easy and cheap; at the far end of the building is the entrance to the car park under the building (with staff to guide you to parking slots), shown on Google Map as "Parking Plaza". There's a lift to street level. Parking costs about €3 for a couple of hours or so.