It's difficult to say from pics...could be hard water stains, or it could be that someone has cleaned it with something that removed the finish, in which case you'll never get it clean.

I have posted many times about this product for removing hard water stains. It's a miracle; I think if it doesn't work on your shower, nothing will. It's safe to use on fiberglass boats, so won't harm your shower. I got a sample from Ace Hardware, so if you have one near you, check there and see if you can get one to try. If not, I order from their website:

https://www.bio-cleanproducts.com/ace-hardware/

If you get the stains off, you can wax the sides of your shower with auto wax to reduce future staining. https://designsbystudioc.com/how-to-wax-a-fiberglass-tub-or-shower/

good luck!

Olychick: thanks for the update on Bio-Clean❣️

Try a mix of equal parts heated white vinegar and blue Dawn. Spray on, let set 30 minutes or so and scrub it off. Good luck.

If you are able to remove the stains, you can do a couple things to prevent them from coming back. No use of real soap in the showers, use liquid body wash instead. Some of the white buildup may be soap scum. Second, get a nice big cloth - chamois or waffle weave or microfiber - and wipe the walls down after every shower or the last shower of the morning/evening.