It may well be when and how you close it. I've used a mesh safety cover for years and this was the FIRST time I had a bloom at all, and not even a bad one. But we had a lot of warmish rain and I couldn't open for weeks.

I usually open to clear water and lots of dead muck on the bottom. You have to get your water ready for closing if you don't want to face this again.

BTW, I live in Northern NJ--similar climate.