Well, 'ex' is probably your former partner. They might be watching your stories out of curiosity. Maybe they still care about what's going on in your life, or they could be just nosy.
No, it's not a very common phrase in general language. It's more of a specific statement that would be used in the context of talking about an ex - partner's activity, especially in relation to digital content like stories on social media.
It could mean that your ex - partner is watching stories. Maybe on social media platforms like Instagram or Snapchat where there are short - form stories available. It might imply that they are still engaged in some form of digital media consumption, perhaps for entertainment or to keep up with friends or trends.
Your ex not watching your stories could be due to a variety of things. Firstly, they might be avoiding any emotional triggers related to you. If the break - up was difficult, seeing your stories could bring back unwanted feelings. Secondly, they could be following a self - imposed rule to distance themselves from you for their own mental health. And lastly, it's possible that they simply don't use the platform as much as you think, so they miss your stories without it being a conscious decision.