That is urates (uric acids) or urine mixed with poop. They can become hard when stuck. Trim carefully and wash off with warm water and a little soap. Droppings stuck to the vent can attract flies that lay maggot eggs, and that can be very dangerous. The chicken has had a bit of diarrhea, which they can do when drinking more water than usual. Does she act sick or lethargic? Is she eating and drinking.