How to remove green dot on iphone camera

That is a naturally occurring effect, called Lens Flare. It is caused by a bright light reflecting off the camera sensor. Before you took the photo the flare was visible on your screen. Often, simply reangling the camera relative to the scene you are taking a photo of, can eliminate the capture of lens flare.

Here are two photos, demonstrating this:

In this photo the lights on my deck caused lens flare (green dots floating above the lights:

Simply angling the phone differently relative to the scene and the lens flare green dots were eliminated: