Alberto, it appears to be fungus, (maybe purely accumulation of algae) but the fungus if present will only be a secondary problem. This fish has dropsy (scales protruded with eyes bulging a bit) and the bacterial infection has spread so far.
In view of the quality of fish, I would put it out of it's misery, which will also prevent the infection and fungus from spreading to your other fish.
There are no real treatment for dropsy although some people feel that you can possibly treat effectively with Virkon S and Acriflavine with salt ??