Beary, Mbar is our 'resident stain expert' and may contribute to this post. I used to deal with these by placing a tri-chlor puck on them (or a satchel of ca-hypo) to bleach them out (this was ~12 yrs ago when I worked for a co that did mostly shotcrete/ marcite finish pools). As I understand it, high pH will allow metals to stain quicker (I've always assumed that acorns have iron in them - just from the stain they 'leaf' ). I also believe that high chlorine will quicken the staining, so I'm not sure why your stain appeared so quickly (??) Until one of the 'stain experts' can get back to you, try the concentrated chlorine I described (puck or satchel), it always worked for me. They may recommend ascorbic acid, but I'll leave that to them (or if no one else answers, I'll give it a shot, but I'm not really sure how it works - just that it does).
Looks like you need to either cover that pool or cut down the oak trees
If you could give a full set of chem #s (cl, fc,cc,pH,TA,CH and cya) it would help everyone who wants to give you advice! Best of luck and lots of fun with your pool! We are here if or when you need us! - Waste