The easiest way to edit these pages would be to create your own small popup page and use the Open Page microflow action instead of the show message microflow action.  In this way, you’ll have complete control over what appears in the popup page.  To specify the text in the popup, you will need to create a non-persistent entity with a string attribute that is used in the popup page.  Before opening your popup page, create an object of the entity with the desired message and use that as a parameter in the Show Page action.