Stain on pool Ladder and Skimmer.....

[Rubber Duck]

We have an 18 foot above ground pool. I shock once a week, and make sure the chlorinator has a 3 inch chlorine tablet in it. Our water is crystal clear!!
However I just noticed that the pool ladder and the skimmer has a light brown stain on it. There are some spots on the pool walls where its shaded that I have what looks like a brown algae. But in very small amounts. I can take the vacuum head and brush it off easily. Like I said, the pool water is crystal clear!

What could these stains be?

I've tested the water, and all chems list as being very low or almost non existant.

[oleja]

I had the same problem and all chemicals were good. While I was at the pool store, another customer overheard my problem and told me I needed acid. He actually took me to his truck gave me about 1 1/2 pounds (dry) and told me to sprinkle it over the affected areas. I did and it worked within minutes! Skimmer and ladder parts are white again.