What causes water to become “milky” at times?

Milky white water is almost always caused by air in the water. One of the many properties of water is its ability to dissolve air. But the water is under pressure in the pipes so the air remains in the water. When you open the faucet and fill your glass with water, the air is free to escape from the water as bubbles, giving the water a milky appearance for a few minutes. Fill a clear glass with water and set it on the counter. If the white color is due to air, the water will begin to clear at the bottom of the glass first, then gradually to the top. This is a natural phenomenon and is completely normal. Your water is safe to use.