These characters will appear in place of spaces if the text was originally written in another application, such as Microsoft Word, and then copied and pasted into the Visual Editor tab. The reason for this is that the original application does not use the same hex value to represent spaces that the HTML Editor does. To correct this issue, you'll need to switch to the Code Editor tab and manually delete the squares. To expedite this process, use the Find and Replace function by going to Edit > Replace.