I had to replace the main 3" waste pipe under my manufactured home because while trying to snake the pipes one fell and broke the T. Inside I found several inches of a white substance that I have checked with vinegar in case it was calcium and used some liquid lightning (basically sulfuric acid) which also had no reaction. The material is white and almost resembleing cheese curd but pretty tough and had to be dug out with a garden trowel. Any ideas of what it might be.

Is it possible you have been washing paint brushes or doing any drywall work in the past? You are at a point where is doesn't really matter what it is. The drain isn't working and, if it fell, the whole drain line should be replaces with PVC. I am also not a fan of using acid, it may have contributed to your problem. Be sure it is all rinsed away before doing any further replacement.

[chuckles]
It is grease