That's a HV discharge. If it's inside the CRT there is nothing you can do. It has some air and it'll either keep doing it or get worse. If it's the flyback transformer developing internal short it's also bad, and nothing you can do. It will fail eventually.
But it could also be dust buildup and/or damage on the HV cable. This you can fix. Remove the dust and dirt from the HV cable and the back of CRT (just do not wash off the aquadag paint!). If you spot any damage on the clean HV cable, like dark pin-pricks that suggest the insulation is breached there, put a 8-10 layers of insulating tape over it. Maybe try to bend the HV cable to avoid any metal/grounded surfaces but be careful not to damage it. Rotating the cup at the tube a bit is usually enough to move it. There might also be plastic rings that are supposed to keep it away from things, but badly placed, those can be moved over the cable to a better spot.
