If the white stuff is very superficial and easily rubbed off, it is probably fungus. Take it out of water for a short time (they don't do well with prolonged drying out like many species) and paint the lesions with Betadine solution and leave dry for a while. Then, in your aquarium or in a separate treatement tank, add Fungus Cure (Marine Phameceuticals - widely available where tropical fish are sold) at the concnetration recommended. The capsules have an extra ingredient that the liquid form lacks. If, however, the lesions are yellowish or tan and appear to be invading the the epidermis, you may have SCUD, a very lethal infection that pignoses are especially subsceptible to. (Septic Cutaneous Ulcerative Disease). This is bacterial and usually requires injectable antibiotics to cure (such as amikacin or Baytril)- otherwise the animal can develop septicimia and destruction of its internal organs if left untreated.