Well, 'ex' stands for ex - boyfriend or ex - girlfriend most likely. They could be the first to watch your stories because they are still curious about what's going on in your life. It might be that they haven't fully moved on yet and seeing your stories gives them a sense of connection to your world. Or perhaps they have a habit of constantly checking social media and your profile just happens to be one of the first they look at.
Ex might be your former partner. Maybe they still have an interest in your life, so they are always quick to check your stories.