Metal in the water (Fiberglass 35K pool)

[blk133]

Hey guys!

After my stain returned I thought I could ignore it, but today it looks so bad that I can't ignore it anymore. In addition to the metal on the walls and floor now I have these spots that don't come off with the brush. I don't know if it's a metal or something else, but when I drop ascorbic acid on it it desappears.

Does ascorbic acid removes algea? Because chlorine couldn't remove it.

My Chl=13 PPM I tried to keep it in that range during 3 days.
PH= 7.2 PPM
Total Chl= 13 PPM
CYA=25-30

I am attaching picture of this ugly stain.

Please throw in some ideas.

Thanks,
B.