How are the reagents (expiration dates)?
Your pool had algae at some point, before this breakout (you said you found it under the stairs) and maybe you've had it all along. Maybe it wasn't actually dead. I've read here about SLAMs that wouldn't end and the advice for that situation is to start tearing things apart to find where the hidden beast lies: stairs, ladders, behind lights, under drains, in the skimmer, etc. And that's in a pool at SLAM level for days/weeks. I'm just speculating in your case, but it is possible that you can have a good FC level and still have algae hiding somewhere. And it's also possible that something is amiss with your CYA number, which would compromise your calculations for a safe FC level. Maybe a few birds used your pool for a toilet Friday. Sort of a perfect storm scenario: you were right on the cusp of an algae outbreak, with your standard FC keeping it just at bay. It's been there all along, you released an additional load of it from under the stairs (or a few animals did their thing), that tipped the scales, and it took off. Just a possible theory, but it doesn't really matter, does it? You know what to do.
And the algae on the stairs, dead or otherwise, doesn't rule out it may be elsewhere, too. Think about where else it may be hiding and have a look see.
Moving forward, you might rethink the stairs. Using them at all. Or maybe up their maintenance: pull them out more than once a year and clean them well, often enough to negate a repeat of this situation. Or is there a model that allows better circulation around and through them? Maybe up your target FC level a notch or two, as well. They're only guidelines after all, each pool is different. And maybe yours is one that needs some extra FC (or CYA).
