This can be aquarium salt, kosher salt or, as a last resort common table salt. Many external problems can be readily cured this way. Do not regularly add salt to Discus water, as is the recommendation by many for community tanks. It does, however, work well as a cure. It will dissipate with your water changes.
Many medications are available for external parasite problems.
Too many to list here along with the problems they cure. This may be their problem. Hexamita This is a fairly common ailment with small Discus. It is an intestinal parasite, commonly brought on by even the slightest stress. If the fish look dark, have white, stringy looking white feces and are generally listless, this may be the problem.