It is called Ick. You can buy medicine for it at any pet store, or even wal-mart. its not too serious, but if it goes untreated it can kill them. It can also cause fin rot if untreated. Make sure you get the medicine, treat them, and after a little while of recovery, clean the tank real well.
and DO NOT put them in fresh water, it causes the fresh water. just clean the tank. fresh water for a salt water fish is is no no. its worse than the ick.