Going by your posts where you posted on the 19th that you started dosing Maracyn 2, you wouldn't have been dosing the full 5 days of treatment yet.
It is VERY important with antibacterial medications that the full treatment regime be completed so you are not helping to create bacteria resistant strains.
Don't be in too big of a rush to replace the seahorses to the display tank as in the marine hobby, "haste makes waste".
You have to be sure you have your water conditions under control in the display tank before you return your seahorses to it.
The most probable cause of the sore is bacteria which increases exponentially for each rising degree temperature above 74°F and is exacerbated by the bacteria being fed by, and providing a medium to grow on, from excess food and seahorse wastes that are not removed from the tank or handled by clean up crew.
Because you are new to the hobby it is possible that you are feeding too much for what they need.
Also, because seahorses are normally very picky eaters, a lot of what you put in the tank will not be eaten just because it doesn't look appropriate to them so it has to be eaten by the clean up crew, and if not, vacuumed out of the tank.
Excess food is usually trapped in and around the rockwork but can also be caught up in decor and when unseen is not removed, causing your type of problem.
As Greg already mentioned, the H. kelloggi coming into the hobby are known for this type of problem anyway, you need to be more diligent about housekeeping than ever before.
