by clay colored do you mean light, almost white colored? Clay can mean different things to different people depending on where they live.
Transit time affects stool color as well as what is in the food. When there is bleeding in the upper GI tract it will cause black tarry stools.
In humans, white chalky stools are caused by gallbladder blockages. This causes a lack of bile thus light colored stools.
If there has been a significant change in your kitty's stool color and you have not recently changed his/her diet, it is sure worth a call to the vet. It could indicates a serious issue, better safe than sorry