That is a sample point, showing colour properties in the sample point dock. Which you do not have showing.

see: https://docs.gimp.org/en/gimp-sample-point-dialog.html

You get them by ctrl-(drag from the ruler)

You can hide them from the View menu

You can drag them off the canvas using the color-picker tool. Hover over the sample point until it changes color, click and drag off the canvas.

These only exists inside Gimp, saved with a Gimp .xcf Not there when you exported a file.

a 30 second demo https://i.imgur.com/5fTxrwg.mp4

rich2005

thanks for the clear explanation and the attached video.
gimper