It might be a guide, which you can remove via Pages View menu : Guides : Clear all guides on page. Its the right color for a default guide. It is not something you see associated with an included Pages template.
If the first paragraph does not clear it, then it might be a line shape that has been made a section master object, and you would have to visit the Pages Arrange menu : Section Master : Make Master Objects Selectable. Then click and remove the line, and revisit the same sub-menu and click Make Master Objects Selectable again to disable it.