It's difficult to determine from a pic, but either a fungus, or Columnaris. Fungus grows on dead or damaged tissue. So if you think the fish scraped itself on the decor, fungus might be the problem. The worst case scenario is Columnaris, or Saddleback. It's a very serious bacterial illness. If you determine the problem is in fact Columnaris, get the sick fish out of the tank. Either euthanize or medicate with a full spectrum antibiotic in a seperate QT tank.