I have a few questions for you – first, what type of filter do you have? And do you have iron in your water?

What it sounds like is that you have iron in your water, likely well water or in extreme cases corrosion can cause metals to corrode and introduce iron into the water as well- and when the pH goes higher, it pushes iron out of solution, water turns green, and at higher doses like shocking the metal will deposit on pool surfaces and cause staining. The stains can be green or brown. Adding a product that is ascorbic acid based will remove the stain from the surface of the pool, but it puts it back in the water. Unless you take the source iron out, it will keep happening.

We have found the only product to bind the iron together large enough that a filter can take it out is ferri iron tablets. The catch is if you have used another product to try to remedy the iron it sometimes prevents the filter from being able to take it out. FerriTabs work best in sand filters, where the iron can be caught and backwashed out. you know the tablets are working because the backwash water will be brown. If you have a de filter they can be used with caution – bumping and backwashing will be needed every 4 hours or so, and cartridge filters will be the least likely to work well with the tablets.

Another product we like is seaklear which will take out trace minerals from water, and can also be used after treating with stain free, and ferritabs.

Copper based algaecides can sometimes cause staining, more often green. The same color green can be seen if you have corrosion of your heater core as well.