Grass carp don't eat duckweed, or watermeal which you may also have. Tilapia supposedly do, but for a pond that small it would be quicker and much more surefire to get some Whitecap which will definitely get rid of both DW and WM.
I would tend to agree with Esshup's recommendation of re-doing the dam and digging the pond out. You could expand it to make it larger than it's been previously as long as the watershed will support it.
