Need Help identifying what this is

[dleeg5199]

Hello, new pool owner here and I need help to determine what these lines and spots are in our plaster. The round dark spots are mainly in the deep end. The thin lines are in our shallow area mainly. I think the round spots are black algae. I do not know if the line like formations are a type of algae or something else. I have put Vitamin C tablets over them and nothing comes up. Chlorine tablets make it a little lighter after alot of scrubbing. Are they algae also?

Thanks for you help.

 

[Casey]

The first pic looks like rust to me.

 

[Patrick_B]

I agree. It looks like something made of steel laid there and left a rust stain.