documentclass[]{article}
usepackage{filecontents,pgfplots}

begin{filecontents}{myContour.csv}
x,y
800,200
50,200
50,0
1000,0
1000,200
900,200
end{filecontents}

begin{document}
pgfplotstableread[col sep = comma]{myContour.csv}{myContour}
begin{tikzpicture}
begin{axis}
addplot [fill=green] table [x={x}, y={y}] {myContour};

end{axis}
end{tikzpicture}
end{document}

addplot [fill=green] table [x={x}, y={y}] {myContour} closedcycle;

addplot [fill=green] table [x={x}, y={y}] {myContour} -- cycle;

documentclass[]{article}
usepackage{filecontents,pgfplots}

begin{filecontents}{myContour.csv}
x,y
800,200
50,200
50,0
1000,0
1000,200
900,200
end{filecontents}

begin{document}
pgfplotstableread[col sep = comma]{myContour.csv}{myContour}
begin{tikzpicture}
begin{axis}
addplot [fill=green] table [x={x}, y={y}] {myContour} -- cycle;

end{axis}
end{tikzpicture}
end{document}