Could be finger oil, could be
fog goo or other contaminates from the air, could be gass leaking out of the
fixture amongst a few options.
See any finger prints? Tell Tale sign but not always there. Could have been someone using a dirty sweaty shirt in thinking it cleaner than the
hand in holding the
envelope to the lamp.
A photo speaks a thousand words and in refining what it is...
For now, have a look at the filaments of the lamps and explain how extensive and where this white stuff is. Also how thick in crustation further. Could mean a lot in what it was in for instance oil reflecting but crustations or dirt not persay and instead absorbing.
Filament blown away from or towards that white stuff? Bottom of top of the lamp as opposed to middle of the lamp? IN a ring about the lamp
etc. White crusties are often a science in themselves.
Oh' and by the way, these lamps are bad and in need of replacement.
Try some 575w/115v Philips/GLA, 750w/115v GE/GLE for long life or most brands of GLC, Osram HPR or Ushio SPH than GE/GLD, or Philips #6981P lamps for more efficient lamps if re-lamping. Should have improved efficiency. Though the EHG isn't that bad of a lamp, it can be improved on.