Does the skin look like its peeling of and then white underneath? It could be due to trauma to the skin like infection. I would strongly consider doing a pH, temp adjusted FW dip with methylene blue and then place in a QT tank and treat with maracyn or some other broad spectrum antibiotic. Dont wait too long because you may reach the point of no return and the fish will die.
Many people will argue against this comment, but I believe in being aggressive when it comes to treating sick fish. More fish are lost because of a wait and see attitude. Puffers tolerate FW dips very well and most antibiotics are tolerated very well by fish. If you dont have a QT tank at least do the FW dip which may buy you some time till you can buy a 10g tank and heater with a small filter or power head. Even a rubbermaid tub willl do.
By the way, I checked Wetwebmedia, Aquarium, Pond, Marine and Freshwater Fish, reef tanks, and Aquatics Information and other people have had this happen to their PP puffers. _________ 75g reef with Nova Extreme 8 bulb HO T-5 lighting, refuge, Remora skimmer, DSB, Seio powerheads, Acro's, LPS corals, assorted zoo's & mushrooms, Yellow Tang, Maroon Clown, Blue Cleaner Goby , Cleaner, Fire and Sexy Shrimp, Coco Worm, T.Crocea Clam, Derasa Clam
120g FOWLR, 260w Orbit power compact lights, Remora Pro skimmer, 30g DIY Sump/Refuge, DSB,Seio Power heads, Powder Blue Tang, Purple Tang, Flag Fin Angel, False Eye Puffer, Copperbanded Butterfly and a Moorish Idol  | 
Sick Puffer? How bad is it? 
