Did you find a solution to this?
No you can't turn this off, the test image signal doesn't come from your grabber but from the HDMI to RCA converter.
The only thing you can do is alter this in WebUI at signal detection section, so the grabber doesn't pickup any "empty" videosignals to Hyperion.
(activate signal detection option) then settings is Hmin Vmin Hmax Vmax to look at. Search on this forum for correct settings.