What's this icon that appears when cycling through the crop tools?

Hello!

I've been using Photoshop for about a decade. This little icon that appears in the top left of my canvas after cycling through all the crop tools has always been a mystery to me:

No matter how much I zoom in, the icon remains the same size on my screen. I can't seem to get rid of it without restarting Photoshop. What is it, and how do I disable it?