Welcome!
No picture yet, so hard to diagnose. Growing algae can be pretty sticky and hard to brush off. But a metal brush is pretty aggressive, so should have had noticeable impact.
It may be some kind of metal staining, staining from organic debris sitting on the bottom, or other.
But first - we need to know much more about your pool, in addition to getting the picture posting figured out.
On this site, over on the left, go to "Settings" and then "Signature"
List out: Pool gallons, rough surface dimensions, ingound or above ground (brand/model of above ground) type of construction (plaster, vinyl liner, etc), added features (spa, waterfall, etc), specific make and model of each of your equipment pieces- pumps, filters, heaters, salt generator, etc. When you get it, which test kit you are using. What form of sanitizer you primarily use (kind of tablets - if any, liquid chlorine, etc.) if no salt generator. You can always go back to edit things if you need to add or change it later.
Get the test kit recommended above. Download to your phone the Pool Math app. You can even link that app to your username here, so we can see all of your entries and then avoid having you retype everything in the responses here.
As to the stain - let us know if Leslie's has had you add anything special somewhat before it appeared - algaecides, clarifiers, phosphorous removers, metal treatments, etc.
