Thank you very much for the answer Jay. Yes, it is just on one side and the rest of the body seems perfectly fine. I will try to take a look from behind, but as you may know, bicolor blennies are not a "stand in the open" fish.Welcome to Reef2Reef!
Weird - it is just on the one side? This isn't a symptom of any of the "classic" diseases, so that's a good thing! Due to the length of time it has had it, and as long as it is still eating, I wouldn't treat it. The only other thing is that there is a parasitic isopod that can get into a fish's gill chamber, where it eats away at the gills. Try to get a view from behind the fish, when it opens its gills to breathe, if you see a dark plated thing living there, it could be that. I have never seen those on a blenny, but I did a quick check and they have been recorded from blennies before.
Yeah, I can move it to a QT. Which antibiotic do you recommend (and how to use it)?It seems to be spreading to me, especially towards the back. Is there any way you can move it to a QT and treat with antibiotics? If not, a distant second choice would be to mix kanaplex with focus and its food, and dose it orally.