I think the Jack's Magic process works, but it can be VERY expensive on badly stained pools.

And, even if you use their process now, with the iron in your fill water, you're going to need to learn to manage the problem over the long term. Do this:

1. Get the Jacks' Magic test samples, or not. Your choice.
2. Put your pool info (volume, pool type(gunite), pool equipment make / model) in your signature (link in MY sig for that)
3. Read the test kit info page (link in my sig, again)
4. Get an OTO kit, ASAP.
5. Order a K-2006
6. As soon as you are able to test pH, begin lowering it with muriatic acid (instruction page link in sig)
7. See if what you have is residue from 'sand algae':

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Before clean up: