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This is probably discharge from the anal glands. There are two openings at the 10 and 2 o'clock position..the discharge can be a light brown...but sometimes the glands become thickened and irritated. The discharge can actually look thick and white.
If you notice the cat grooming the backend more than normal or scooting her rear end...the glands need to be checked.
If she is not bothering it, you don't need to do anything. If you see it again, or the cat seems bothered, a vet will express the glands.