Man... I sure wish I could grow greenwater/phytoplankton like that.
My cultures crash easily, usually right after I get it a good gatorade green... I do know that it crashes because creatures that eat it contaminate the jars, if I want to restore it I sieve these critters out and then lower the salinity. So I guess hypo could have allowed greenwater already present in your tank to thrive, because it killed or knocked back something that is eating it. In my case it's rotifers and other plankters, don't know about your case. I started my cultures with DT's phytoplankton. If you use that then well, that's probably what it is. Great food for some coral, clams and feather dusters.
The few times I have seen greenwater explode in a tank it's been in the presence of extremely high phosphate. Last time that happened I had my friend bring a tapwater sample. It was over 160 out of the tap! No wonder she had algae. Also 40 ppm nitrate... you using tapwater by any chance?
I've got no degree in anything so am no expert, ... especially since I am pretty good at killing greenwater. Adding rotifers to your tank would do the job but your filter would probably take them out.
Use a lot of aeration because the ph could fluctuate as the green produces massive amounts of oxygen during the day and consumes it at night. Also, when it does die do a massive water change because it will lead to an ammonia spike. Usually the dieoff is sudden.
I agree on the refractometers. I would take the new one back to the store for a refund as it's likely not accurate. Plastic refractometers are terrible, every one is different. And they cost WAY too much. For the price of 2 you can buy one real refractometer off ebay. There is a lady in Seattle who sells them for a good price and she is honest.
Kate