I'll try to help the best I can. Ann is MUCH MUCH MUCH better at treatments and ill seahorses than I am. But I do have treatment info and the handy, dandy treatment book for seahorse diseases that was written by some very knowledgeable people. Until Ann shows up, this is my opinion...... (Ann, feel free to correct anything I type if it's wrong or outdated treatments)
First, it's very important to check the rest of the body - especially the chest and pouch areas for any signs of discoloration from the norm. If it's a 'flesh eating' bacteria, symptoms sometimes appear elsewhere on the body.
Since the tails of seahorses drag about the substrate, rock, etc and are rather sensitive on the undersides it's possible there was a simple cut/scrape that is now infected - or as Teresa mentioned it could be the beginning of tail rot.
Tail rot is usually seen in tanks that are kept too warm (ie: above 74F). I won't go into the whys and hows because I could talk till I'm blue in the face and people who disagree with me will still argue LOL.
At this point, as Teresa mentioned, I would get the seahorse in a 10 gal QT tank and lower the tank to 68F to slow the reproduction of the bacteria. If you feel it's an infected scratch, you can apply Bio-Bandage as a topical treatment to see if there is any improvement and check for improvement or worsening over the next couple days. If your tank is normally running a bit on the warm side, I would tend to say it's probably not a scratch. In that case, I personally would still use Bio-Bandage but I'd also treat the QT tank with Neomycin and Triple Sulfa.
Neomycin Dosage:
Day 1:
Add 2ml of liquid Neomycin to 1 cup of hospital tank water. (or 400mg of powdered Neomycin)
Pour into a high flow area of the hospital tank (ie: filter output, etc).
DAYS 2 – 10 of Treatment
Do a 50% water change each day. After the water change, add 1 ml of liquid Neomycin (or 200mg of powdered Neomycin) to 1 cup of hospital tank water and pour into a high-flow area of the hospital tank.
Triple Sulfa Dosage:
DAY 1 of Treatment
Completely mix the medication with about 1 cup of hospital tank water. Use recommended dosage on package for 10 gal tank. Pour the mixture into a high-flow area of the hospital tank.
DAYS 2 – 10 of Treatment
After the daily 50% water change that was already done when dosing the Neomycin...completely mix packages recommended dose for 10 gal tank with 1 cup of hospital tank water. Pour into high flow area of the hospital tank.
Note: DO NOT STOP TREATMENT FOR ANY REASON (whether it appears to be working or not) FOR THE FULL 10 DAYS.
P.S. - Get that frogspawn outta the tank!