White Down Arrow on Laptop Screen

Does anyone know what's causing the white down arrow on my MacBook Pro screen. See attached image. It remains stationary, no matter what application I'm using. It was previously near the top of the screen and recently moved toward the bottom. I'm using an external monitor, connected via a USB-C-to-HDMI cable. The arrow only appears on the laptop screen, not the external monitor.

Any ideas what it is or how to get rid of it?

Thanks.