I am not sure,from that photo either...It is hard to get a MI to stop long enough to pose for a photo.The entire vent looks a bit swollen,however,I don't know if it is cause for alarm.How long have you had it? Is it the only MI in your aquarium?Is it eating well? IMO leave it,unless it appears worse,and/or, your MI is appearing stressed,give it a little more observation.