Sorry, no pics (yet) - I'll try to get some later today.
Stains are in the deep end of a plaster pool. They appear to be rust in color, blotchy and varying in consistency and dime-quarter sized. There are several of them. Along the drain it's like whatever formed them mostly settled along the edge of the drain and forms a larger 1/4 semi-circle on the plaster right next to the drain. (Could this be a fertilizer stain...?)
I don't know how long they've been there as my friend just moved in.
Did a D&F as the CYA was well over 200 (now it's 45-50), and before adding Cl I tried scrubbing with first with a Cl puck, then some Vit C tablets to no avail. Had to finally add Cl last night as it's going to be over 100 here for the next several days...
Any ideas are appreciated.