[BayouRunner]

It's a combination of grease, soap and everything else that goes down your drain. We have had it a few times over the last twenty five years in my place. In our house it only happens between the kitchen and the septic system. Looks exactly the same as what you have in your picture. It will completely clog the pipe. It's easy to knock through. I put a clean out right before the septic tank and used a commode snake. A drain machine would probably do a better job. It easy to dislodge though

 

[BayouRunner]

I retrieved the same kind of debris (a little whiter in color) from AC drain. Very odd, I expect the AC condensation water to be pure. After many days of pouring in vinegar, wait 24 hours and vacuum piece of this kind of debris, I still cannot open up the pipe. I have tried CO2 pressure gun, suction, water pressure....

Ac drains are definitely not pure. You can get some pretty nasty stuff in there. When it's real bad we just replace the entire drain pipe. Most of the time we use co2. Sulphuric acid works pretty well if pipe replacement is not feasible. We only change the 3/4 line, nothing larger. But at times it even stops up the main line 1/12 or 2 inch pipe. We get the homeowner to call a sewer service at that point.

 

[BayouRunner]

Is there some special trick to preventing it from reoccuring? Like using hot water in the washing machine instead of cold or regularly flushing with baking soda and vinegar or just boiling water down the kitchen sink? I've spent over $500 on two plumbers in less than a year and this substance is what they tell me is the issue but they are no help at all on what it is so I can stop it happening again?

Have no idea really. I’ve not been able to pinpoint it myself