What is this?

[sunofthebeach]

stupid question - but have you tried pouring a 50/50 water bleach solution on it and letting it set? It could be like mildew - does it stay wet there all the time (like from a mister or water feature)? I get something like that on my concrete walk-ways where there is moisture and little direct sun.

I tried to power wash it no change, and I have poured bleach on it, but i did not really scrub it. There was no change. It does not stay wet.

[sunofthebeach]

I have noticed a couple other spots that are developing around my pool.

Can anyone say it is not black alge?

[sunofthebeach]

My pool builder was out today. The black staing on my coping is Iron Ore that was in the flagstone when it was harversted. It leaches out over time.

they are going to replace it for free as it is defective material.