Well, there is rare and then there is rare. We hear about this several times a year, but it doesn't begin to compare to the number of times someone says their pool is green and it is actually algae. So it isn't rare in the sense of "I heard of that happening once ten years ago" kind of way, but it is rare compared to the number of times a pool turns green because of algae.
The two, algae green and metals green, are often mixed up on the forum, even though they look totally different. People just say green and it isn't obvious what they mean by that. So, if someone says their pool is green then it is 99% that it is algae and only 1% that it is metals, which is one kind of rare.