Looks like diarrhoea. This can be caused either by chickens eating too much, heat stress, antibiotics, worms, coccidiosis, infectious coryza, too much protein, viruses....the list goes on. But I wouldn't worry to much. Some times chickens just do funky poops. I'd be on the lookout for: blood in poop, drowsiness, not their normal self. If they start acting strange please let us know. Other than that, don't sweat it.