It's a white square with a red line through it. Click the rectangle inside a rectangle option at the bottom left of the menu that appears.Click the Shapes button on the left of the menu.
How to adjust border color with the Markup editor
How to draw on your photos with the Markup editor.How to find the Markup editor in the Photos app.
That means you can quickly take a snap shot, add an arrow, circle an area of interest, drop in some text, and then send it to a friend, co-worker, or post it up on the internet. Markup is Apple's extension for adding text, callouts, shapes, and more to images and documents and, with macOS Sierra, it's now built right into Photos for Mac.