That's from trapped chemical fumes Susan. You're likely losing the "moisture barrier" (aka plastic bag) too, which means it's going to start getting heavy.
In your case, at a rental, I can't say it's avoidable. As you've learned yourself, rentals are tough on spas.
You might buy another year or two by removing the foam cores (note how easy that sounds?) and re-wrapping in a plastic sheet, but it's going to start dropping debris in the water as well.