What is wrong with your pool if you get a white powerdy build up on your fingers when you rub the walls of your fiberglass pool and after using the pool for the day you find the water becomes milky?

The gell coat is oxidizing or mineral deposits are building up. Find an approved method of polishing the gel coat that will remove oxidization and mineral buildup. If the white stuff is under the surface of the water your alkalinity may be unbalanced. If you have a D.E. filter you may have a small leak. You should brush the pool and make sure the filter you are using is working properly vacuuming will also help.

3rd answer:

I believe the first answer would be the better one. The second would be unlikely based upon the entire question above.

K