I may have the unpopular opinion here but it looks like a sting/abrasion to me. Tail rot moves very very fast and IMO if that's what your female has it would be quickly moving up her tail by now.
What else do you have in the tank? Corals, fish, crabs, heater, etc - anything that could of hurt her tail? Sorry if you've already answered those questions already, just trying to get the full picture.
Since tail rot can be brought on by small injuries I would treat her with betadine dabbed on her tail a couple times a day. Just hold her out of water and softly dab the area with a dry q-tip then apply the betadine, wait 20 seconds or so then blot dry and release her. Even better if you can find Allicin in the first aid section and do the same as the betadine 3-4x a day.
Hopefully she wont get full on tail rot. Keep us updated!