How long ago did she injure her tail?
Because of the whitening of the tail, I would move her to a separate tank where you can keep a close eye on her (and where she is away from the other seahorses to protect their health). I like bare bottom 10 gallon aquariums. If you don't have a spare cycled filter, you'll need to do 50% water changes daily, and remove uneaten food and waste daily, to prevent ammonia. Make sure there is plenty of gas exchange. I like to place several open airlines in the tank.
Bring the temperature on the aquarium that you put her in down to 68 degrees. This will slow, and possibly stop, the growth of a bacterial infection.
Also, if you don't yet have it on hand, I suggest you purchase Furan-2 or purchase neomycin and triple sulfa. They are sometimes available through your LFS, and can also be purchased from
www.seahorsesource.com Depending on your answer to the "how long ago did she injure her tail" question, you may need to start treating for bacterial infection, and you'll need them on hand if you need to. Topical neomycin gel would also be helpful.
If you could get a picture, that would be really helpful.
Is she still eating?