Are you using maracyn I or maracyn II? I believe that the majority of bacteria related popeye cases are caused by aeromonas bacteria--this is gram negative bacteria. Maracyn I is for gram positive bacteria, so it probably will not help. However, since you have started medicating with it, you must follow through with it according to the instructions. It is safe to use Maracyn II, which is for gram negative bacteria, while you're using Maracyn I.
I suggest researching aeromonas bacteria and I always suggest anyone using antibiotics to read this article: http://edis.ifas.ufl.edu/fa084 you will use antibiotics more wisely and with greater efficiency if you have a better understanding of how they really work. Good luck.
The thing on the back that says it treats everything under the sun is only intended to get you to buy the product. The maracyn II box says exactly the same thing as the maracyn I box, iirc, even though they treat different bacteria. Some cases of popeye are caused by different bacteria--but I believe that particular kind are usually found in pond fish. If he looks like he's doing really well, you could go through with the maracyn I and see what happens, then switch to maracyn II later if you're not seeing results. This all depends on how he's doing, though, so it's definitely a judgment call on your part.
It's a really good sign that he's eating, so you should definitely be happy about that. Keep his water super super clean and follow the directions on the packaging of the antibiotics exactly and he should recover.
Phew. Well, I noticed the "white eyebrow" of popeye a week or so ago... the eye isn't bulging, but I recognized it immediately from a rescue fish that I tried to cure of popeye, and began treating a day later when it didn't go away overnight. He always eats, although his poop has been odd for a few days. I have antiparasite medicated food, but he won't eat it. I fasted him for days and he still spat it back out. :c
The only change in his behavior at all has been that he no longer hides as much from me as he did a while back, he swims forward to me when I come to his tank and he doesn't scare as easily as he used to.
It's odd, though, he only has it on one eye. And it's honestly hard to tell if the eye is bulging, he moves too much. But it doesn't LOOK like it is... I just thought I'd nip this in the bud if it was something bad...
Well, it's odd. It looks like normal feces but with a white mucous coating. I haven't been able to observe him going these past few days, but earlier today I saw what appeared to be white fuss in his anal area, and a while later (after I put in the maracyn) it's completely gone. I thought perhaps this was poop clinging to him, although I couldn't tell.
I talked to some people about internal parasites, and they said that if he had them he would stop eating. I got really concerned, but so far he refuses to eat the medicated pellets and scarfs his usual food whenever I give it to him.
Usually when a fish has a bacterial infection they have a noticeable behavioral reaction--such as hiding and lethargy, you said your fish was actually coming out more than usual, and his appetite clearly hasn't been affected. He's probably fine. You should still finish the treatment of Maracyn-I, and proceed with the antibiotic food according to the packaging too.
As for the poop--you could give him something extra fibrous to eat like frozen brine shrimp or daphnia to get more observable feces. It's kind of difficult to tell if what you're describing is a symptom or not. In the cases I've experienced with intestinal parasites, though, the fish did lose their appetite.