Looks like iron staining. Get a bottle of vitamin C pills and place one on the step. You should see almost immediate lightening in a much larger area than you might expect. If it lightens it, that conforms iron staining and you will want to do the same thing on a larger scale with an ascorbic acid treatment Ascorbic Treatment to rid Pool of metal stains then use a sequestration regularly to prevent staining.
Here are the directions to use Ascorbic acid to rid your pool of metal stains: Here is the ascorbic treatment: Take your chlorine down to 0 and PH down to 7.2, if there is chlorine in the water it won't hurt, it will just eat up the ascorbic acid, so you will need more to get rid of the...
Ascorbic acid removes the stains and places them back into the water. You must then continue to use a sequestrant to keep the metals from falling back out of solution and turning to solids that stain your pool surface.
Ascorbic acid lifts the stains.
Sequestrants keep the stains lifted.
The only way to remove the metal from the water is to exchange it. You must keep using sequestrants as they eventually get destroyed by the chlorine/degrade over time and thus need to be replaced to keep the metal from turning back into a solid and staining your surface.