What is this spot? black algae?

[albel1214]

I have a plaster pool. Never have had any issues with algae. I was brushing the pool floor tonight and noticed this spot that would refuse to brush off. I even flipped the pool brush over and pushed the hard side against the spot and nothing.

Is this black algae? I zoomed in very close. The spot looks to be about the size of a half dollar.

 

[YippeeSkippy]

It almost looks to me like something metal fell into the pool and rusted.

 

[Donldson]

Difficult to say for sure from a picture but based on your description it doesn't look or sound like algae. Usually algae will brush away, especially that size.

Looks like tar to me, has any construction or roof work been going on? That is just a shot in the dark of course. You haven't dropped a penny or any chlorine tablets in near there have you?

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