Just a temporary fading of their markings from being in muddy water so they blend in with their environment. I think I have heard this referred to as the "chameleon effect" (even though this term was originally created to describe social behaviors of people moving between different social groups.) I have noticed this over the years while fishing different water colors, especially muddy conditions.

Have you ever put a fish (especially a trout) in a water filled cooler? Many times their markings will fade quickly to "blend in" or "wash out". If you are looking for examples of this, there are plenty. Deep water vs. shallow water fish is a prime example. The amount of available sunlight hitting them seems to be one of the driving forces in determining the clarity of the fish's markings.

I have noticed this ( with muskies) most often during the algae bloom on the South end of Chautauqua or when catching them in muddy water here on the Allegheny.

It would be interesting to catch the same 'clear water' fish during the day and once again at night just to compare the subtle differences (if any).