Fish "flashing" can be caused for several reasons, predominantly parasites but water quality can cause the skin to become irritated.
As mentioned by BBReef
, a proper <<quarantine tank
>> should be prepared in the event the fish have a parasite problem. If necessary, they can then be treated preferabley with <<hyposalinity
Have you noticed any other problems, white sugar spots on the fish, labored breathing, darting or surface gulping?
What are the water parameters, tank specs and fish?
Moved to the SW sick fish/coral section