What are these black spots?

[Ithacarun5]

These showed up the morning after adding my non-chlorinated shock (MPS). I think our fill water is iron heavy, related? Or is this algae? Doesn't wipe off with bleach, but does with muriatic acid. Again, these showed up after adding shock, so I'm thinking a metal got oxidized? Had plenty of a bromine bank once I shocked.

 

[Shirker]

If you attached pictures, they are not showing up for me.

 

[Ithacarun5]

If you attached pictures, they are not showing up for me.

 

[JamesW]

Didn't you already post this in a different thread?

 

[Shirker]

Showing up now, but that doesn't look like classic iron staining to me. Does it have a texture? Slimmy? Sticky?

 

[Ithacarun5]

The black spots seemed to be armored, lol, a melamine foam and vinegar wiped them away, with elbow grease though. I don't get how it could be algae/mold. The bromine bank was high, where it should, when I went to shock with MPS, then the black spots showed up by morning. I had the water tested this afternoon and I was advised to use Metal Gon before shocking. So I now have the tub emptied, black spots wiped up, and kind of stumped what to do next. Fill it up again with line flush and drain? Or can I just fill it up now and try the Metal Gon?