I'd just give him hay and water for a day or two and see if it goes away (it should), and then start re-introducing the veggies one or two types at a time.
It could just be that he's sloppy -- the cecals are coated or walled with a mucus, they usually swallow them whole, but he could be breaking them or dropping them before he eats them, or they could be slightly runny.
I really think it is just cecal residue. If there's just a small amount, nothing to worry about, but it is most likely diet-related.
Try experimenting and see what makes it better or worse.
If it was urine it would mean he was 'leaking', and rabbit urine is rarely the same color all the time, so that's more doubtful.
Let us know the results!
sas :clover: