The greenway can be really annoying in the summer because of the crowds and careless riders who use it. The other rider should have looked before turning, besides there is no guarantee yelling "on your left" would have prompted the right response since some people get spooked and move toward their left. I usually just slow down and pass with a lot of room to anticipate the actions of the careless. I had the same thing happen to me a few months ago and the other rider had the nerve to blame me for not anticipating what he was going to do, so I asked him "at what point are you responsible for anything?". Moral of the story is to plan for the actions of the careless.