About 7 years ago my surgeon found on hiatal hernia the size of a quarter. Now takin 2 prevavid daily. Why I experience clay colored stools w ab pain?

Pancreas. Sounds like it is not your hiatal hernia, rather a problem with your pancreas, possibly chronic pancreatits. See your primary doctor to get started, and you may need to see a GI specialist, or ultimately a surgeon if that's the case. Good luck.
Not related. The hiatal hernia is a classic cause of heart burn so you taking the Prevacid (lansoprazole) certainly makes sense. The abdominal pain and clay colored stools suggest that you need to be checked out to make sure that your liver is working properly. Start by seeing a primary care doctor and they should check you direct bilirubin level and any other appropriate blood work. Good luck.