Sometimes their poop just looks off and it isn't necessarily a problem. Keep an eye on it for a few weeks and if it continues to look different, then having it tested may be worth while. Usually when their urate is off color then it has to do with
hydration levels. Offering some extra water by dripping it on his nose to lick up may be helpful. I wouldn't worry about it too much at this point though, if it was just like that once or twice.