Is it only in your Moodle site -not on other sites? Is it all of Moodle or only the editing?
If it is only on your Moodle site and not others, I've seen this happen when some text has been added to a page and was copied and pasted from Word or from another website. Also it could be a block that was recently added and affecting the page styling.
Try removing anything you have added to the page recently until you notice a difference.
Try removing anything you have added to the page recently until you notice a difference.