My stool was a whitish gray for about 18 hours and now has turned to a grayish yellow. what could be the cause?

Changed stool color: Color of stool varies with foods, speed of emptying of GI tract, medications & supplements ingested, gut flora (unique for each individual), additives (like blood), subtractions (like absent bile), infection, inflammation, & other factors. Best case scenario for your specific changes: mucus discharge; worst case: partial biliary obstruction.Unless you correlate color change to a food, get checked.