Lack of bias to the Grid causes over current cathode to anode.
Bluing or plasma discharge is by Ionized air, probably given off by overheating.
The Vacuum tube or Thermionic Valve as we like to call them
It now highly suspect
Ionized-air glow
From Wikipedia, the free encyclopedia
Ionized-air glow is the fluorescent emission of characteristic blue–purple–violet light, of color called electric blue, by air subjected to an energy flux.