Thanks for the response. It's on the underside and she's an outside cat. The more I look at it, it looks like a scab. I'll monitor it daily and if it gets worse I'll take her to the vet.Hi, welcome!
A couple questions, does she go outside? Is this on the upper side or under side of her tail? If inside only, are there other pets?
I'd leave it alone and just watch that it continues to heal. You might continue to take photos every day or so to track progress/shrinkage.