Can I suppress the dot under an image caption in pages for macOS?
This issue occurs when you choose the "Image Gallery" option from one of two places either:


Menu Bar > Insert > Image Gallery
Tool Bar > Media > Image Gallery 


The dot underneath is used to support multiple images being in that box similar to an image gallery function on a website.

The reason that you do not see this when you copy and paste an image or drag one in is that it inserts the image as an image and not as a single image within an image gallery.

The solution is to delete the image gallery and insert the image into the Document by one of the following ways:


Drag and Drop from Finder onto the Pages Document
Copy and Paste the image
Menu Bar > Insert > Chose ... (Select file)
Tool Bar > Media > Chose ... (Select file)


I have added an image to further illustrate this incase it was not clear.