Shoot. The returns should have been plugged by the plaster guys to avoid this possibility. Always fill the spa after the pool. At least you figured it out fast enough.
All you can do now is finishing filling the pool. The finish is too tender to walk on, so there's no working on any stains from inside the pool. Brushing is about all you can, or should, do.
So fill the pool as fast as possible. More hoses if you got 'em.
Then have the PB take a look and ask him to address this. IMO, it was his responsibility to either plug the holes or manage the fill or both. If he just put hoses in the pool, down low, and you took it upon yourself to fill the spa, then he's less to blame. Though he should have at least warned you not to fill the spa before the pool was done. Now if he filled the spa, then this is on him. That aside, you shouldn't try any DIY fix because 1) you could make it worse, and 2) you could give the PB a good excuse to put this back on you, as in "Well, you shouldn't have done that, now it's your fault."
Hopefully, there will be no stain, or at worst a faint one and/or he will agree to try and mitigate it. Their go-to move is usually to apply acid to stains. This doesn't really attack the stain, but rather burns off a thin layer of plaster taking the stain with it. Sometimes that can leave the surface etched. Not exactly rough, but not as smooth as it should be. So that's not my favorite solution.
Very mild abrasion is preferred, IMO, because it is more controllable, will remove the minimum of plaster required, and leaves the finish smooth. But there will be a point of diminishing returns, when the abrading causes more of a visual scar than it removes the stain. Plus it's labor intensive, and especially so underwater. I'm not sure what is the best abrasion material to try. Not a steel brush for sure, that will be too harsh. We'll cross that bridge if we have to.
Or you can do what I did when I burned an acid stain in my brand new finish on day two. I accepted it as a mistake I made because I didn't know any better and c'est la vie. Every pool has its birth marks. If yours has only one then that in itself is pretty much a miracle. If there is a permanent mark (let's hope otherwise), it's likely that only you will ever notice it. No one has ever noticed mine. Live and learn.
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