Black lumps/spots on steps - turn green if you rub them

[rpoldervaart]

I have a few black spots showing up on my top step. They are a raised round spot, looks black, and if rubbed, I can see it smear green. Is this black algae? It doesn't seem to look like pictures I've seen others post. Just a few raised 1/4 inch diameter spots here and there. I don't think I've ever had anything but FC = TC and no CC, but yesterday noticed a slight bit of red tint when testing for CC. My FC got low and I didn't notice right away.

FC 1.5 - usually kept around 4-5
CC barely .5
pH 7.5
Alk 110
CYA 60
Salt 3200
I don't remember CA right off hand, but think it was 260 or so.

I was going to raise the CYA back up to 80 (we've had a lot of rain and a lot of water has gone out the overflow), and then set the pool up with borates. But figure I need to tackle these black spots first. Shocked last night with 6% bleach to about 17 FC. I didn't think about leaving it high for a couple days, so it's probably dropped some today in the sun while I'm at work. Will scrub the spots with a wooden brush tonight. I guess I need to keep the FC high for a couple days, brush the spots, and see if they go away.

How high do you think I should bring the FC?

Thanks,
Robert
21000 gal, in ground, salt Aqualogic PS8

 

[JasonLion]

That does sound like black algae, more or less round black spots that are slightly slimy and when a bit is scraped off with a finger nail and smudged onto a white paper/card shows green.

Try semi-gently (do more than pat it, but don't grind it down) rubbing one of the spots with a trichlor tablet and see if it will go away that way.

 

[Isaac-1]

I am wondering if your top step may be part way out of the water, allowing it to trap water then let the sun drive off some/all of the chlorine thereby giving some type of algae a place to grow. I have had a similar sort of problem due to condensation on my brass shut off valve for the pool fill.

Ike

 

[rpoldervaart]

Sorry for the delay and thanks for the responses.

The step is about 4-6 inches below the surface. The spots tend to be toward the wall, so I'm guessing there is very little circulation right there.

I bought a trichloride puck and rubbed it on the spots. Kind of hard to say whether I rubbed too hard or not, but I tried to rub without grinding the spot away. The spots turned brown in a few minutes. I'm going to try to see if they will brush away now.

I've retested the water and have the following:
FC 4.5
CC 0
pH 7.5
Alk 80 (lowered it by acid / aeration - going to go down to 70 and then adding borates)
cya 80 - added stabilizer
salt - 3400 - added some salt

Seeing that I don't have any CC, I guess I'll just spot treat the spots with a brush and the chlorine puck, and see if I can't get it cleaned up that way.

Thanks,

Robert