Did he eat yellow crayons??? Geez, that just does not look right at all. Did he get into something when you weren't looking? How old is he? How long has this been going on?
Give it a few days to see if it clears up. If not I would take a stool sample to your vet. He could have an over growth of bacteria in his gut and they can give you stuff for that if the fecal comes back positive.
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