No matter how careful you are, unless you completely cover everything in your bathroom before painting, you're likely to sprinkle paint on surfaces such as your fiberglass shower stall or bathtub. Perhaps the paint was left behind by a previous painter. If the paint is fresh, you're in luck--you have a handy water source available, and soap and water will remove fresh paint quite easily. If the paint is old, you'll need more than soap and water, but there are solvents that will clean up old dried paint without harming the fiberglass.
Close the shower door and let the shower run with hot water only for several minutes. If the shower drain has a stopper, close it and let the shower stall or tub fill with hot water and let it sit for about 15 minutes, then drain it. Heat, steam and hot water will soften paint if it's less than a week old.
Scrub the softened paint with a fiberglass-safe abrasive sponge, or a rag made from an old towel. Re-wet the paint frequently. An abrasive cleaner that won't harm fiberglass can help with stubborn dried paint.
Apply a generous amount of denatured, or rubbing, alcohol to the paint, and rub it with a rough rag to remove dried paint. If the paint has dried and cured, this will take several applications. Also try commercial latex paint removers, which are as effective as denatured alcohol, although more expensive.
Dip a rag in acetone or lacquer thinner and use it to scrub away dried paint. Most nail polish remover contains acetone and will work well to dissolve paint without harming fiberglass.