#contagious

Had this Blenny for about a week now and I check on him daily, today I noticed this large ball on its fin. Any ideas what this could be, should I isolate it is it contagious, treatment ideas? Anything helps photos below thank you!

Hey all, after a pretty depressing trip to the LFS yesterday for advice, I've decided I have to tear down my 20 gallon nano reef because of an absolutely terrible colonial hydroid infestation which I have been battling for close to six months now to no avail. I'm going to salvage what corals I...