I really don't know what benefits duckweek has, but I always put some in my boxie's little pond and also my Cherries pond. I've never seen my RF eat any, but the boxies love it.
It has the highest average protein content of any plant and contains rather high concentrations of oxalates depending upon the calcium concentration of the water; up to twice that of spinach. I'm sure in small amounts as part of a varied diet, it would do no harm. Many animals, including water turtles (and humans), eat it.