sorry could you explain this a bit more, it doesn't make much sense. Rain is caused by evaporated water, nutrients (and any dissolved solids) dont evaporate, its why saltwater tanks need topping up with freshwater and water changing with salt water. Rain may collect extremely small amounts of nutrients as it condenses and falls but how could it possibly contain more nutrients than rainwater that has runoff down a catchment, been held in a dam and then pumped to your house?? even if you test the TDS, kH, phosphates, metals, whatever and compare rainwater and tap water, the tap water will have more. Regarding your green grass, wouldn't that be more likely due to the volume that comes out of your hose compared to the sky?
There is not enough info to give "the answer" but these things happen, algal blooms occur and die off very quickly, in commercial aquaculture, especially hatcheries that use ponds put alot of effort into culturing their plankton blooms. They actually develop different fertilizer ratio's for the different plankton communities that occur at different times of the year.
It could be a bunch of things, even something as simple as the pot is now getting more light for some reason.