What is the green ring at the top center of my iPhone.

It appeared on my phone temporarily when I unlock my phone and then disappeared. This happened for a minute or so every time I closed my phone then opened and unlocked it again. I closed out of all apps and it still continued. I set my phone down to try and find an answer using my iPad and nest time I picked up my iPhone it wasn’t happening anymore and I haven’t noticed it since. I’m using an iPhone 14 pro max and I have iOS 17.3 now but it was the update before this one.