Aerators are strainers attached to your faucet or showerhead that break up the flow of water as it leaves your tap. Aerator screens can collect particles found in water and should be routinely cleaned throughout the year and replaced once a year. Particle buildup is often white and comes from a variety of sources.
The most common source of buildup in aerators is from the hot water heater. The hot water heater dip tube is made of a nontoxic plastic material called polypropylene. This plastic can break apart or disintegrate and travel in the hot water to your faucet, eventually collecting in the aerator.