Copper algaecides are a common cause for pool metal problems.
Unfortunately, we have not been able to develop a consistently successful method of removing stains, and resolving problems with metals. So, while I can offer some advice that will probably help, I can't guarantee there all the stains will be removed.
Let's start with some basics.
1. Cleaning up, while the water is still cool and algae less of a problem, is a good idea.
2. You DEFINITELY need a K2006 kit, in order to manage things well, as you clean up.
3. Let us know whether the stains are a current issue for you -- getting the metal out is easier if you aren't too fussed about a few copper stains. For what's it's worth, those stains do tend to make the pool a bit more resistant to algae.
4. While removing metals, you do NOT want to rapidly raise the pH OR 'shock' the pool. Both changes can cause the rapid formation of stains.
5. Likewise, while removing stains and metals, running your pump 24/7 will help. But, if you can, you should adjust your eyeball returns so that water returning from the filter is directed out into the pool, and not against any pool surface.
6. "Metal-out" products do NOT actually remove metals from the pool; instead they TEMPORARILY dissolve the metals and KEEP the metals dissolved. But once the products break down chemically, the metals are released and tend to reappear. Ideally, they reappear on the filter and are removed with the filter is cleaned.
7. So . . . (a) order a K2006 kit (see link to t-kit page in my blue signature bar), (b) report test results once you have them, (c) avoid shocking the pool, (d) keep the pH low-ish (7.2 - 7.4) for now, (e) tell us what's important to you: removing all metals AND stains, or just avoiding recurrent problems this summer, (f) tell us if the mustard algae has been a recurrent problem, or was just a 1x issue.
Best wishes.
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