I'd guess more likely to be bacterial in nature based on the picture and your description of the area of concern.
If it were my seahorse I'd be placing it in a ten gallon hospital tank and lowering the temperature to 68°F.
Most likely I'd treat with Furan II.
If it does end up being gas bubble disease it will need to be treated with Diamox.
You should also consider improving your husbandry practices for the display tank as bacterial diseases (and yes, gas bubble disease also) most often are a result of water quality.
Unfortunately, this water quality is not something you can measure with any hobby test kit.
Uneaten food trapped/hidden can decay, producing food and bedding for nasty bacteria like the vibrios types, and it can cause chemical imbalances within the seahorses organisms that can lead to situations like liver disease and gas bubble disease.
These conditions are exascerbated by the seahorse feeding method that masticates the food when they snick it up, passing minute particulate matter out through the gills and into the water column, further degrading water quality.
I would do more frequent and larger water changes, vacuuming any trapped and hidden detritus in the process.
