Hi Sigrid,
It's definitely not phragmites, but it looks like a variety of Miscanthus sinensis, which is also known as Chinese or Japanese silvergrass like you guessed. Here's a pretty extensive info page about it: https://www.fs.fed.us/database/feis/plants/graminoid/missin/all.html
It seems like some varieties are more aggressive than others, so it's hard to know what this one will do. I haven't been able to find any reports of it being invasive in Michigan although it's reported to be aggressive at least and invasive in other parts of the country and judging from the size of that pa