Fish will "flash" for a few reasons. Parasites is one obvious one, ich and there are many others. Someone wrote on here that ich first attacks the gills so it is not seen; if conditions are stable and the fish healthy, they will fight it off. I have this in my aquaria periodically, sometimes even to the stage of seeing one or two spots on a fish. I monitor it but otherwise ignore it, and full-blown ich never develops and it goes away, fought off by the fish's immune system. But the water conditions can negatively affect the fish's abilities in this, so they have to be checked.
Which brings me to water, flashing also occurs due to toxins like ammonia and nitrite, etc. Also bacteria and protozoan infections. And I have known food to get stuck in the gills, especially on corydoras feeding on worms and such.
Unless you are aware of something that might be connected, in which case taking appropriate action to remedy it, I would suggest monitoring the fish carefully, looking out for spots, blemishes, patches, etc., and noting the increase or decrease of the flashing.