Mysterious green-shaded text boxes in older file

I've opened a file from six years and at least one version ago only to find that all the text boxes are shaded green with green strokes. Further, all of the linked files appear purple! The files don't seem to have any problems: all the fonts are good and no broken links. New text boxes don't have the green, and the green doesn't follow the text when it's copied into a new box.

They all go away when I select Overprint Preview.

What's going on here?!? Something obvious, I assume.