Your pool water is not very clear yet but you didn't ask about that.
First, you have to clear the green algae off the steps. brushing into chlorinated water is the best way.
Once the green is gone, you will need to identify the stain.....it is probably iron but we need to make sure.
Put another pic back up when the green is outs there and we'll do a Vitamin C test to confirm it's iron and then go from there.