The white stuff is probably plasticizer leaking out of the plastic tubing on the wires that has gathered white dust or the same stuff 'knob fungus' is...or in short old plastic being normal old plastic.
A lot of sets the internal wiring and sometimes the cord will get sticky from plasticizer....my favorite pair of Sansui classique 1500 tower speakers have a wood grain vinyl covering that continuously releases plasticizer...I clean it and in a month or so it is back... leave it long enough and it gets hairy with dust bunnies and lint.
If only the HV reg circuit was malfunctioning then the CRT would still load the HV...If the CRT was cutoff or it's heater was dark you probably could not get over 25KV out of it...I don't think mine could do more than 23 with the CRT dark and the reg pulled. A brand new clean set should not make HV snap at 25KV, but an old set with conductive dirt arcing paths, degraded insulation etc that has never seen more than 19KV that is now suddenly getting say 23KV is a horse of a different color...
The snap may have been HV. If it only happened once there's no sense in worrying about it, just troubleshoot it as though you have no memory of the set before today and it magically appeared in your house (it's more fun that way too...look santa brought me a CTC5!
) . If the sound reoccurs regularly track it down with avengence.