The most common cause of grey stool is a problem with the liver, pancreas or gall bladder. The fact that she is not eating well and is 10 gives more than enough reason to seek having blood work done to determine if one of the aforementioned organs is not functioning properly. It is also common for cats who are not eating well over the 3 day mark to develop a condition known as fatty liver disease (hepatic lipidosis) which can result in liver failure.
If the litter she consumed was clay-based, this may also have contributed but the presence in the stool should have happened 24-48 hours after the consumption. If not, it's likely linked to something else.
Until you can have her seen, I would recommend trying pureed baby food in chicken, turkey and similar flavors. Avoid those that contain onion or garlic in the ingredient panel. This may entice her to being eating again and, if so, you can slowly switch her back over to canned food and eventually kibble, as well. Her lack of eating may be due to nausea, and it won't hurt to administer a dose of regular pepcid (famotidine) every 12-24 hours. You will want to give 0.5mg/pound of body weight (a 10# ***** would receive 5mg, a 5# ***** would receive 2.5mg, etc). For this, you can visit any human pharmacy and buy the OTC brand name Pepcid, or you can use the cheaper, off-brand “famotidine” that’s available. Either will be useful.
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