hmm, i had a similar problem with one of my female guppies, only her case was different. it made her gill stick out almost as far as it could without breaking off. there was some almost bubble-looking growth or something growing out of it. none of my other fish got it from her. i still to this day have no idea what it was. probably some sort of parasite but the situation that youre talking about doesnt sound like mine exactly. that bubble on your cory might be a growth or something or might even be a parasite of some kind...