I think you've got a custom, non-factory supplied item there, as I've never heard of one before.
But it's a good idea if the anchor shank bangs down hard on the deck, say by the windlass, and can't be eliminated, although I've not had that problem.
There are also chain and anchor shank locks and other devices found on the pages of marine catalogues that are designed to control anchor shanks and chain rodes. Using one of these may be a good replacement if you can get the plastic sheet off.
If you want to replace the one you've got, you'll have to buy a piece of Starboard or a panel of HDPE (high density poly ethylene) (as found in cutting boards) and cut a replacement out yourself and then also get the old one off.
Try prying it off gently. If that doesn't work, try heating a flat piece of steel, like a putty knife, and working it a bit at a time under the plastic sheet, then breaking or cutting the sheet out in sections. But be very careful NOT TO BURN OR DEFORM the fiberglass underneath.
The best way would probably be to pass a hot wire between the plastic sheet and the fiberglass, melting the adhesive, but would require some careful experimentation and probably two persons working together. You'd need some nichrome (nickel-chromium) wire and a large variable 120 volt AC resistor like a POWERSTAT to control the voltage, and therefore the heat, developed by the wire, and you'd have to keep it stretched tight between the plastic sheet and the deck. It works like a heat powered band saw, and the system is often used to cut plastics, but under carefully controlled conditions.
Best to start out simple and see how you can get the job done more simply.
Maybe others will have some ideas to help out.
Good Luck!
Joe. :teeth :thup