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This can be a sign of a urinary tract infection or it might be due to something he ate as some of the food dyes end up in urine.
This may also be due to increases in bilirubin, indicating there is a liver or kidney problem or another health issue.
Bilirubin is a byproduct of hemoglobin (red blood cell) breakdown. High bilirubin levels may indicate an increase in red blood cell destruction.
I'd plan a vet check up for him if you continue to see green urine from him.
Hope this helps you!