White poop is not poop, it's actually urates. It's part of what they expel when they pee. The thick runny liquid sounds like a runny stool. I would clean it out and watch for the next one and if it is also runny, consider taking a sample to the vet for a fecal test. But fecal samples need to be fresh to be tested, so try to get it there ASAP after it happens.