I lived in Pismo Beach for a year and for one week-long period I think I saw it every day. It's subtle and hard to differentiate from just going blind looking at the Sun.

I have been lucky enough to see the green flash myself recently, having been on the lookout for something like 20 years. It was definitely not subtle. It was a bright green flash, startling and breathtaking, lasting for a couple of seconds. If you have seen it, you know you have seen it.