Pour a pot of boiling water onto the area and let it sit for a few minutes. Then use a plunger to suction onto the top of the plastic and pull until it pops off.
You should not use anything that will cause the paint to dry on the outside of the tub, such as a paper towel or a cloth. You should also not use anything that will cause the paint to come off in pieces, such as a sharp object.
There is no one definitive answer to this question. Different shower surrounds may use different types of fiberglass or acrylic, so it’s important to research which one is right for you before purchasing. Some good sources of information on this topic include bathtub manufacturers and home builders.