The material is a refractory cement, and I can’t explain the stringy characteristic, except it may be reinforcing fiber (fiberglass, asbestos) to hold the mortar together. Refractory cement is a high temperature mortar that is made by combining a refractory clay with a conventional cement mortar mix. Its easy to make and replace. Buy the refractory clay and standard mortar. Use about 20% clay to 80% mortar and mix with water, pack it into the opening, and allow to set. Get rid of as much of the old crumbling stuff as you can, and to be safe (reduce dust), remove it wet.