The green flash is something used to impress new sailors, or at least those who have never sailed Atlantic offshore. It can also be used to keep morale up through bad weather, if the person does not know that it does not happen in a cloudy situation. Kinda gets everyone together just before sundown, makes for great dinner conversation, especially if you are expecting a fairly bad night and the weather is due to be bad for a few more days.
It does have to do with light refraction, I would expect. I have seen it 16 times now. Each time there were absolutely no clouds from horizon to at least directly overhead. In all the times I have watched the lovely Key West sunsets, and sunsets from the islands, rum punch in hand, I have not seen it from land.
When it does happen after you have played it up to people who have heard about it and think it''s not real, it is fantastic. Makes it even better.
So don''t stare yourself silly. If the weather is right it will happen. On the other hand, if you''re having a yucky trip and the people onboard have only heard a bit about it, it is a great conversation topic - diverts worry about the weather.