I agree with you, I doubt it's it. My mollies are starting to get it too, and they are most definitely not stressed. It's the alpha male that's starting to develop this coloration brightly. My water conditions are perfect in a brackish tank. I don't think anything is wrong with them. Don't know about the one that's flashing, might want to look at something other then the coloration as an irritant. Let's keep watching and keep each other posted please.
P.S. Maybe it was the wood. That would be my thought.