Bicolor Blenny hole on side

[reefer108]

I have a bicolor blenny in my reef tank and there is a pretty large hole on his left side just behind his gill. There isn't any aggressive fish in my tank so I don't think its from a fight. The hole isn't deep enough to show any organs so He still eats and swims good. I have been treating it with a powder that I got from the fish store but it doesn't seem to be getting much better. Does any one know what this might be? I will post a picture soon.