From the Minnesota DNR website:

 

What is starry stonewort?

Starry stonewort are grass-like algae that are not native to North America. The plant was first confirmed in Minnesota in Lake Koronis, near Paynesville in Stearns County, in late August of 2015. Plant fragments were probably brought into the state on a trailered watercraft from infested waters in another state. In August of 2016, starry stonewort was confirmed in several north-central Minnesota lakes in Beltrami, Itasca and Cass counties. DNR invasive species staff found that a number of other reports of suspected infestations were native species that appear similar to starry stonewort.

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More information:

 

Starry stonewort fact sheet
Starry stonewort watch card
Starry stonewort lookalikes

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