Melted CRT anode cap
The large "hat" over the connection is supposed to be pliable, so that it seals against the glass and provides a long "creepage" path that's free of dust, moisture, etc.

The problem is that the plasticizers used in the rubbery material either leak out or break down over extended periods of time, leaving the cap either very brittle or "glued" to the glass &mdash; which seems to be what you have found.

Yes, the loud noise was probably an arc-over of some sort.

The best fix would be to make sure the tube is completely discharged. Use something sharp that can push directly through the insulating cap to the terminal at the center. Then, scrape off the old cap to the best of your ability and clean the glass with alcohol in order to remove any remaining contamination. Replace the rubber cap with a fresh one.

Only then will you be able to determine whether the high-voltage power supply was damaged by the arc.