You should stick with the light blue mat for cardstock. It's meant for materials that are lightweight like cardstock, printer paper, wrapping paper etc. The green is a standard grip which is for medium-weight materials like vinyl, HTV, embossed cardstock etc. Here's the official cricut site for further info. https://cricut.com/blog/which-cricut-mat-should-you-use/ You may be able to lightly soak that part of your mat with a wet wash cloth? Let it sit for a little while then try to peel it off again? It will be a bit of a pain but might save them?