It is usually grease combined with soap, not mold. Cooking fats and oils cool in the pipe and react with minerals in the water, forming a waxy, white, soap-like deposit. It sticks to the pipe wall, catches food and hair, and narrows the drain until it backs up. Hot water only moves it further down.
Fats, oils, and grease go down the drain as liquid and cool within a few feet. In the pipe, they bond with calcium from the water and with dish soap, a chemical reaction that turns them into a hard, soap-like material. That is what shows up as white clumps in a kitchen sink or a basin trap. Each layer builds on the last.
DFW tap water carries enough calcium and magnesium to feed this reaction, and kitchen lines take the most grease.
Not every white deposit is grease. A hard, crusty ring at the drain opening is usually mineral scale from hard water, and fuzzy growth in a damp bathroom drain can be biofilm. Grease clumps are soft and waxy and come back after plunging. Chemical cleaners dissolve a little, but they sit on the clog, and bleach does not help.
Nuflow DFW is a licensed trenchless sewer contractor serving 25 Dallas-Fort Worth cities, RMP# 46694. Hydro jetting scours grease off the pipe wall, and a camera inspection shows how much is left. You keep the footage. Call (469) 701-0597.