I have had blennies come with internal parisites before, if it is eating well but still struggling, look care full at its belly. It may need deworming.
+1 If it has been eating any type of food added to the tank "like a pig" ... whether it be flake, pellet, mysis or whatever else you may have been feeding, my guess would be it's an intestinal parasite starving the fish of nutrition.
That said, you mentioned you were feeding it flake every other day. Try feeding it a bit of flake and/or pellets, mysis, etc. twice a day. (Small fish should be fed small amounts two or three times a day rather than large amounts every couple of days.)
Also, watch the poop. light brown 'logs', are often a sign of constipation (kind of normal if they eat a lot of protein foods like LRS Reef Frenzy or Rod's). If it's white, thin and stringy it's often an indication of intestinal worms/parasites. If it's like a burst of mist that disappears within a second or two, it's typically a normal, healthy bowel movement.