With Marine Ich (Cryptocaryon irritans) seeing isn't always believing, as BIG-G points out.
Fish generally flash due to an irritation of the gills. This can be parasitic (worms, fluke, protozoan, etc.). For instance, you may not notice the fish has Marine Velvet (Amyloodinium ocellatum) except for this one symptom. By the time the fish gets covered by the tiny parasites at a stage visible to the naked eye, the fish is usually doomed to die.
At the other extreme, the fish can just be normally flashing. If the flashing more frequent than once every 10 minutes, there is likely a problem. If the flashing is more frequent than once every 5 minutes, there is something definitely wrong.