I can't really tell from the pics... are you referring to the mark near the tail? If so, it very well could be fungus. Unless you're seeing white spots, I would rule out Ich.
Either way, Ich-Attack is a really good med to use for treating both Ich and fungus. If you believe that it's fungus, just follow the instructions on the bottle, remove any carbon from your filter media and do NOT increase the temp, (high temps could make the fungus worse).