That box is telling you that you have too much text to be displayed in the available area. Reduce the amount or size of text and you will likely see it disappear. But remember that if your end users resize their browser down to a smaller size it will reappear.
It is the expansion button, which displays when the text doesn't fit in the text container.
From the screenshot it is impossible to see if you are in a responsive project with Fluid Boxes? If that is the case, try to decrease the minimum font size from the default 14pt to 10 or even 8pt. It may lead to disappearance of some of the expansion buttons.
Thank you both very much. I am in a responsive project, so I'll take those steps.