Green hydrogen just means you've used only renewable energy to electrolyse water. Splitting the molecule into one O and two Hs. The problem is, water is actually quite a perplexing molecule. For instance, most materials shrink as they get colder and expand when they get hotter. Water shrinks as it gets colder, to a certain point. Then as it gets even colder, it starts expanding again! It's also quite tenacious. And there really doesn't seem to be any way of not expending a LOT energy to split it. Really doesn't seem to be much reason to make green hydrogen, other than where you might need it for some industrial purpose (chemical for instance) without putting carbon in the air by getting it from fossil fuels (blue hydrogen).