Hi Yann. You're safe to swim, and as Marty suggested, can't remove sequestered metal. But since your rate is low, over time it should dilute if there's no incoming metal. You may need low, periodic maintenance doses, but you can let your eyes be the guide.
If the product was inclined to lift the stain, you should have noticed the staining lightening by now I'd think, since I am a day late to the thread. Eg. In the past when I used sequestrant if I had mild staining on fiberglass steps, it would lift immediately. But yours being in paster and possibly with scale (because the dry acid made a difference) may not respond.
If it doesn't lift, you've now at least a) stopped it from getting darker and b) now can try to release the stain without worrying that it will redeposit, because as the metals are released, the sequestrant will pick them back up and bond with them.
You're now in a position to try the underwater acid wash wand if you so desire.
Btw, were the stains there since you first took over the pool, or did the stains emerge subsequent to slamming?