Bernice Brewster says

Carp pox, or more correctly fish pox, is caused by a herpes virus, the same group of viruses that cause cold sores in humans. The virus attacks the fine layer of outer skin, infecting the cells and making them produce more virus particles instead of mucus. The response of a Koi to infection with the virus is to produce more cells, which are seen on the body surface as white lumps with a waxy appearance. By the time these waxy lumps are visible, the viral infection has already subsided. It is important to realise, despite all the advances of the twenty-first century, that we still have no cures for viral infections. On the whole, fish pox is not particularly harmful to Koi and therefore the best treatment is to leave it alone. In the past it was advocated to scrape the waxy lumps off, but this damages the skin, leaving it open to secondary infections that are often more harmful than the original infection. Fish pox usually disappears in summer when water temperatures rise.