How to replace background green color
This is a byproduct of the lenscorrection algorithm, originally meant to work on RGB pixels only. If the output pixel has no corresponding input pixel mapped, it is populated with 0, but in a YUV format, that's a shade of green.

As a workaround, you'll have to force manual format conversion before and after the filter.

ffmpeg -i out.mp4 -vf "format=gbrp,lenscorrection=cx=0.5:cy=0.5:k1=0.4:k2=0.4,format=yuv420p" sss.mp4