I'd observe and try to figure out what it is..
-sand
-fungal
-parasite
-ich
If its sand, you'll know quite quickly as it won't stay on the fish long.
If it is fungal, it shouldn't be too bad to treat. Maybe a dip medicinal treatment, return to tank, and feed it well (?).
If you can quarantine and treat with Prazi-Pro for 5-7 days that should help clear up a parasitic/worm problem.
If it is ich... you may need to consider quarantining, treating with copper (curpramine for example), and letting the main tank go fallow, or empty of fish for ~6 weeks. Ick sucks.