Firstly, I would like to assure that my own chickens have excreted this type of fecal matter before. Secondly from what I gather this type of poo is what can be referred to as, "ceacal," or "cecum." These are produced from the caecum of the chicken and are mustard to dark brown froth and they are expelled every 8 to 10 droppings. So, pretty normal. Now if this is continually the type of poo they are excreting, this could be reason for concern.