A couple more points to consider. If this is a relatively new seahorse (say, less than 6 months in your tank), and purchased from your LFS, you may be looking at an underlying parasite issue. Trematodes are common parasites of wild caught and even captive bred seahorses that have been exposed to parasites in a LFS holding tank. If the eyes are CLOUDY as well as bulged, you may very well have a parasitic infestation. If so, first do a freshwater dip to verify the presence of the parasites. It will take a few minutes for the trematodes to leave the fish -- the reading I did suggests 10 minutes. If you see little white seed-shaped objects left behind in the dip water, you have just confirmed trematodes. A concentrated formalin bath is recommended if that's the case.
As everyone else has mentioned, popeye isn't really a disease in and of itself. It's a symptom of something else. Parasites, bacterial infection (though this isn't as likely),gas bubble disease, and poor water conditions all can cause popeye. A good google search for "seahorse popeye" will give you a couple general pointers in additional diagnosis and treatment options.