Agreeing with Albert. An occasionally green urate sounds pretty normal.
As long as it isn't slimy green 'poop' and it has more of a urate like consistency, it's not uncommon. Could be from a bit of shipping stress and the bile. Sometimes urates will even be almost a blue tint or bright orange. Unless this odd color urate is happening all the time and is the only urate, I wouldn't worry.
If it is more like a poop and the stools start getting runny as well, you may need a sample checked for parasites.