I used to have a continueing battle with duckweed about a year ago, then I just gave in and keep it in control with a net, a little bit can look quite attractive.
Not really. Just keep picking away and in time you'll be rid of it. Be warned it is a VERY FAST grower and spreads like fire. I can leave only a dozen or so little leaves and within a few weeks they can take over about 90 percent of the surface in my 30gal if left alone.
It does look good in little sections of the tank and smaller fish as well as cherry shrimp love to hide in the dangling roots. Just keep on top of it and you won't have problems.