If you don't get that off it could block her up and cause other problems. Good that you noticed.
Check to make sure that her "insides are not turned outside". That would indicate Prolapse. Something you should look up just in case for the future.
Hopefully that's not the case in your situation.
I trimmed a hen's feathers back there since my girls have exceptionally fluffy butts and 1 hen had that PLUS prolapse. After the 2nd time, I trimmed her. I felt the hanging stuff was contributing to the problem. She has not prolapsed again. It's easier to break the poo off if it's crusty and then bathe them. It get's very polluted and close to the skin. If you choose to trim - be very careful. Mine went very calm and still when I layed her on her side in my lap.
My girls like a bath occasionally.
Fill up a sink or bucket with warm water, not cold, and not hot, with a bit of dish soap is fine. She should calm down and soak in there for a bit. Try to clean her little bum as best as possible.Then towel dry, and maybe a light blow dry so she's not put away wet.
I anticipated my roosters freaking out when I washed them last, but they actually seemed to like the warm water.
I cant think of a sillier sight to my neighbors than giving my chickens a bath. I dont even wash my truck.
We just got a coupla Speckled Sussex that are almost ready to lay. She wants them gone as she has bought some younger birds and doesnt want size conflicts. These birds were dirty and crusty feet. One had poop on its back, probably from a roost. A large tote was given a 5 gal bucket of warm water, luke warm to huumans. I put about a 1/4 cup of bleach and a tablespoon of dish soap inthe water, mixed around. Moving slowly I put the bird slowly into the water with a two handed football grip around the wings, so she cant flap and panic. So far, I dont have much trouble until I try to gentle spray them off with the hose. It tends to be a little cool, and Mom helps while I hold. I just turn the soaking bird loose, and they look cold for a bit until they stop dripping, and then start to preen themselves. They are mostly dry by nightfall, and a WHOLE lot better looking for it. The Feet are clean too, so I have a chance of stopping any pathogens from hitching a ride, which is the larger goal for me. We have a trip tomorrow, to pick up some more birds close to laying stage, Barred and RIR.
By handling the bird before bathtime gives me the chance to check for any faults or damage, parasites, and general condition.
Thank you all! I have had my chickens for about 5 years and haven't ever seen one with this much crust. They are free range but are confined to our acre back yard by a chain link fence. They go to their coop at night so I will lock them up for easy capture in the am and I will wash her then. Should be a fun experience. Hopefully it is not anything serious.