I'm not a pro, but every time I've seen a signal on a stick obstructing the tracks it's been a "blue flag". Either a metal flag painted blue, or a blue light, or both. I know a blue flag is a safety indicator; I believe it tells employees not to touch equipment that is protected with the flag, and I think that there are strict rules about who can remove the blue flag.
That said, it doesn't look to be a different color from what I think are green dwarf signals alongside turnouts, but the color rendering in the picture is a little iffy.