turning white is usually a sign of zooxanthellae loss and a definate stress indicator. I wouldn't really suspect water flow as the culprit unless sustained for a good period. They will close up pretty quickly but it should not cause that kind of reaction.
Something like this would usually be the result of a chemical problem (recent water change or dosing), substantial change (increase usually) in lighting or coral warfare.
Anything there ring true?
Might also wanna check with reefrunner I think he has them as well.