My white fiberglass tub/shower keeps a stain anywhere the water runs ?

Try installing a water filter to your shower. They make some that attach between the shower head and pipe. Or you can try using a spray like rain-x to the shower so it dosesnt stick, but this might make your tub slippery.

You Can Rent A Fire Torch To Burn Them Out. Or Hoe Them Up Then Burn. You Want To Make Sure Theyre Dead As Well As Any Seeds Or Theyll Just Grow Back.

MAKE SURE YOU CLEAN IT AFTER IT HAS BEEN USED AND WHILE STILL WARM USING SHAMPOO AND A NON SCRATCH SPONGE. There are also Cream cleaners one called SHADAZZLE is supposed to be good

I agree with installing a filter. You may need a whole house filter. Check with your village on what is in the water and advice on the type of filter that you would need. Use Easy Off Oven No Fume Oven Cleaner for an easier clean. Spray it on, let it foam and sit till it liquefies. Then rinse. May need some light scrubbing. You can use Rain X or an auto polish the slow down the build up.