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If it is just fecal matter, you can wipe it off. Just do it gently in case it is stuck in the hair. If this is the case, you can soak the dog in a warm tub to loosen the hair and unstick the dog. This is called a "fecal ball." Dogs can actually get impacted and this leads to bigger problems if this is not taken care of.
You might also be seeing a prolapsed rectum. If you suspect this, you need to see your vet.
Let me know if you still have questions.
I would use a warm, wet cloth and wipe the dog's bottom. If it is stuck, don't pull it. Put her in some warm water and soak her until the fecal ball starts to disolve.
If there is an obstruction preventing the feces from passing, you will need to see the vet.
Let me know if you still don't understand.