How to get rid of the white line on my iPhone photos?

The white moving line is showing you the contours of the main object and separates it from the background. When you see it, you can easily copy the object without the background to paste it into a message or a note.

You can also create a sticker from the object and save it. On iOS 17 you can paste the sticker onto another photo to change the background. See: Make stickers from your photos on iPhone - Apple Support (IN)

If you do not want to see the distracting white snake, close the info of the photo. It will only be shown, when the info is open.

Your iPhone is scanning your photos to recognize the objects in the photos or videos. This process is running in the background, whenever you are not using the iPhone and it is connected to a power source. The object recognition requires a lot of processing. it can take weeks or even month, if you are having a very large photos library. Give your iPhone plenty of time to complete this task by keeping it connected to the charger whenever possible and when you are not using it.

The iPhone may need to repeat the object recognition whenever you install a major system upgrade and the algorithm for the object recognition has been improved.

Not all objects can be recognised or separated from the background. It will depend on how cluttered the background is. On my iPhone with iOS17 I can also extract the main subject from Live Photos. I have just to open the photo to view it full screen and to tap and hold the subject, then release it and tap "Add Sticker"

You‘re welcome 😊

This feature can be fun. I used am using it frequently to exchange the background, for example for this picture of a crane.