Not exactly sure.
First off, there has to be some light source as the scene is much less interesting when we cannot 'see' anything happening. So I'm guessing that the green glow is a plot device. Green, like in Minas Morgul, to indicate decay.
I initially thought that it was some moss or other plant-life that was emitting the glow, and though that might have been possible in Middle Earth, there are no 'green light-emitting' plants, mosses, etc in the real world. However, if the moss on walls contained the
luciferase enzyme, which would be great to have in shubbery, then they would emit the glow. If there were ordinary moss and a little bit of light squeaking though the doorway, we may have a green glow as seen
here.
Discounting that, knowing that the Barrow was littered with gems, jewels and other crystals, we could also consider
triboluminescence, but that's a bit unlikely due to the lack of activity in the room. I would then say that the green glow is somehow an indication of the wight, and Frodo, being the bearer of the Ring and already, pre-Weathertop, not your average hobbit, is sensitive enough to see it. Why is Frodo set apart? Was this do to the Ring or Frodo's resistance? Or being taken last? (Did I remember that right?) Was the Barrow Wight (in the books, not on this forum
) trying to force Frodo down the wrong path? If Frodo would have walked away, leaving his friends behind in the cold barrow, his downfall and the recapture of the Ring by Sauron would have been assured. Gandalf, later in Rivendell, notes that that moment in the Barrowdowns was the scariest of all in the journey from the Shire to Imladris.
Anyway, those're my thoughts.