Black dots in the pool

[Johnklavanian]

hello
I'm trying to figure out what to do about these black dots. they are only in one area of the pool by the ladder. they showed up randomly last week. they do not brush off. I tried using the hth stain protector and treating for algae. I also shocked the pool. it has not changed at all. I do have a new pool vacuum which I thought maybe was scratching the wall? but it's the Polaris 3900 and it blue not black. I'm at a loss, doesn't seem like algae because it won't brush off.
any ideas?

 

[PoolStored]

Use a fingernail, or a bamboo skewer and scrape a few, then wipe on a paper towel. Post a pic of the paper towel.

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[Texas Splash]

A full set of water test results would also be a good idea.

 

[Johnklavanian]

the scraper/my fingernail does scrape it off, but what comes off disappears immediately, there is no debris to wipe on the towel.

here are test results from just now;
FC: 4.2
combined C 0.6
ph 7.2 (I just added 1 cup of soda ash now)
total Alk 60
CA hardness 300

this is from the Taylor k-2006c kit