Though it is difficult to know the exact nature of the white mass floating on top of swimming pool water without a chemical test or evaluation, the mass is most likely some form of mould, algae or microbial bacteria. It is not uncommon for there to be a build-up of mould or algae on top of the water, which can be green, black or pink, when the swimming pool is covered during the winter. When the algae dies, it can turn white. With proper cleaning and chemical treatment, you can remove the floating white "stuff" on top of your pool water.
Remove as much of the white mass as you can with your pool net. When you remove it, shake it out into a trash bag. Don't put it on your lawn or on the ground around your pool.
Scrub your pool walls with a heavy-duty scrub brush. Even if you can't see any algae, mould or other bacteria, scrub around the walls of the pool as thoroughly as you can. Wear a protective mask and gloves when cleaning the pool, as you don't want to inhale or touch any mould spores or algae.
Check your pool's pH levels. They should be between 7.2 and 7.6. If the pH levels are not within this range, chlorine will not work efficiently. Your chlorine level should never drop below 1 ppm. If it does, or has in the past, it could explain why you are getting an algae or bacterial build-up. Add chlorine to the pool if necessary.
Vacuum the pool using a backwash or waste setting. Do not use the filter setting; otherwise dead algae or mould can become trapped in the filter and could return to the pool.
Add an algicide pool chemical once a week during the summer if your pool regularly gets a build-up of algae or a floating white mass.
Never use your swimming pool until algae, bacteria or mould is dead and removed. Once it is clean, do not use the pool until the chlorine level is safely over 1 ppm.