Sounds like ich. Start feeding it food soaked in garlic juice/extract. Don't put in the garlic pieces if you make an extract. You can also buy one by kent and other marine product manufacturers. Another great product, IMO, is zoe and zoecon.
If they can't fight it off on their own, and it gets worse, you will most likely need to perform hyposalinity. Another alternative is copper, but most people try to refrain from chemicals. For info on hypo, do a search, there are tons of threads with the procedure.