Yellowed not white anymore
Fiberglass and acrylic showers yellow as they age.
Can you clean and make it whiter
Unfortunately, that is part of the issue with this type of material. You can try a deep clean to remove a bit of it but it shows up more against some colors and that's probably why you now notice it. Check out wet & forget shower, it might help over time.
You could try spraying it with hydrogen peroxide on a regular basis to see if that helps, but as the others have said, it's normal for fiberglass to turn yellow over time. :( Wishing you the best.
As fiberglass ages it turns yellow. If you think it will clean, I would get Gel Gloss cleaner and shiner. Find it at Walmart and Amazon