A green screen, or green macro-blocking, is caused by having the resolution set to a high value (D1 / 720×480) and the bitrate set to a low value (2.5 MB or lower).
To avoid this issue, either the resolution needs to be lowered or the bitrate increased.
It is recommended for D1 resolution that the bitrate be set to a minimum of 2.5 and even this could be to low depending upon the input video. A video source with lots of action or motion requires a lot more bits to encode accurately than does a talking head, so this needs to be considered as well when configuring the encoder. Another method to get more video bits into a given stream is to disable/mute the audio.