Fish Has Bubble in Dorsal Fin

[boozzbro]

Hey all I have a lawnmower blenny in my 90 gallon tank that seems to have developed what I can only describe as a tumor. He has been in there just shy of a year. I've attached a couple of poor quality pictures of him. Any idea on what this could be besides what I think it is? No livestock changes in the past 2 or 3 months and he is eating fine without it obviously effecting his swimming ability.

SG: 1.025
Amm: 0
Nitrite: 0
Nitrate 1
Phos:0.36
Ca:430
Alk:7.6 dkh
Mg: 1440