How do I turn off the white line effect in photos on iPhone SE?

I'm guessing that you mean the white outline, like this:

When this appears seems to depend on the iOS -- you don't say which version of iOS 18 you're using. The white outline shows the object in the picture that can be separated from the background. You can select that and make a "sticker" from it, or you can copy that part, like the woodpecker above,

for pasting into other stuff. You can also look up information about the object.

On my iOS 18.4.1, the outline seems only to appear in two ways:

• It shows in the Info Window, like in the image above. Clearly, the fix is to not use the Info Window. Normally, the Info Window is used by folks wanting to know stuff about the picture, so an outline of the object identification seems appropriate, there.
• Or if you long touch the object in the picture. like this

where your given the choices of things to do with the outlined object. In iOS 18.4.1. It will disappear if you touch it again.

In earlier versions, the object outline might occur at other times.

If you're not in iOS 18.4.1, you may want to update your system.

Is this what you mean? Does updating fix it?

I'm not aware of a white line effect for photos. Does this occur when you are taking photos or viewing photos?

I am aware of the feature to lift a subject from an image, which when you hold on to them it will outline them in white. Lift a subject from the photo or video background on iPhone - Apple Support

if this occurs when you are taking or showing up in your photos, this is likely an issue with your camera. I suggest you try a Forced Restart and troubleshoot with the Support Article I provided below. If you have tried everything there to no avail, there is likely a hardware issue that needs service at your nearest Apple Store Genius Bar or Apple Authorized Service Provider.

If the camera or flash on your iPhone, iPad, or iPod touch isn't working - Apple Support

Force restart iPhone - Apple Support

If needed..

Genius Bar Reservation and Apple Support Options - Apple