Yes I have seen that. But as they eat more and more they should progress to the regular poo. They are finishing up their egg yolk from what I have read. I usually see the green poos when they are first starting to poo. (I don't know what the books say about it, but from what I have seen brooding batches of chicks it is normal AT FIRST.)
(Green watery poo in an older bird would be a sign of them not eating enough.)
Also, as a side note, every eight poos or so they will have a caecal poo, which is liquid, all their lives. Do a search for the "poo page" on here if you want to see pics from another website of poos.