It depends on the nature of your past relationship and your ex's personality. If they were a bit controlling in the past, then it could be a sign that they are still trying to have some sort of influence. But if it was a relatively normal breakup, it could just be their way of staying casually informed about an old acquaintance.
This could indicate a few things. Firstly, they might be struggling to let go completely. They may be holding on to the memories and by watching your stories, they feel a bit closer to that part of their life. Secondly, it could be a form of passive communication. They're not ready to reach out directly but still want to be involved in your life in some small way. And lastly, it might just be a matter of boredom or nosiness. They have nothing better to do and seeing your stories gives them something to look at. However, without communicating with them, it's hard to know for sure.
If you've already moved on emotionally, there's no real need to worry. But if you're still in a bit of a vulnerable state, it could be a bit concerning. Maybe they are hoping to re - enter your life through this small form of connection. You should assess your own feelings towards them and the situation. If you don't want any more interaction, you could block them from seeing your stories. But if you're open to the idea of being friends in the future, you could just let it be for now and see if they make any further moves.