replying to Resident, Edjustment wrote:
It is called aluminum hydroxide. It is formed as a result of the aluminum
anode (or sacrificial rod) reacting with the chemicals in the water. The
anode breaks down due to the electro chemical process in the water to protect
the tank from rusting. Oddly enough aluminum hydroxide is used in makeup.

--
for full context, visit https://www.homeownershub.com/mainte...in-182489-.htm