When you have suggesting mode turned on, all changes you make in your text will be tracked. Newly added text will be tracked with a green highlight:

To turn this highlight off, you will need to turn off suggesting mode. To do this, click on the "Suggesting" button at the top right of your manuscript:

This will switch you back into composition mode, and the button will look like this:

You will also need to accept the previous changes to make the existing green highlight go away. To do this, click on the "Activity" tab in your manuscript, and then click on the checkmark to accept the change.