This morning I got some flickering red lines on the screen, as I was flying (see below).  I've not encountered this before and at first thought it might be signally a white balance issue (is it?).  I also went into the "anti-flicker" settings, but wasn't able to select Auto or 60Hz, and it remained on 50Hz.  
Can anyone know what this is about?

It's the focus peak. It marks in red the areas of the image that are in focus. You can disable that in the camera options if I recall correctly.

CuaC is correct you can set it to varying levels or turn it off, it is designed as an aid to indicate what in the depth of field is in focus.
Regards,
-d.

Ah - Thanks....
Makes sense.