Black Algae

[duraleigh]

Staining (spots, anything visible) can be either metals (mineral) or organic.

If they are organic, most always they are removed or lightened noticeably with a chlorine tablet placed on the for a few minutes.

If they are metal , (inorganic) they can usually be lightened noticeably by crushing up some Vitamin C tablets and putting them in a sock or nylon then holding it directly on the stain for five minutes or maybe more.

I suspect they are inorganic from the looks of the spot you posted.

Other possibility is simply dark colored stone that used to have a very thin layer of plaster covering it and now that very thin layer has worn off.

 

[boston62]

Well I tried vitamin c, 3" tablet, and MA with a piece of PVC pipe, never touched it, I am thinking that its like you said thin layer wearing off. Thanks for the help...

 

[boston62]

After brushing this spot, this what came loose. It's a stone, what could be making these stones to show now? The pool is 9 years old... Is it possible that the pool cleaner running might be wearing the finish down exposing these stones?