Fish, like humans, can develope what we call skin tags which might just be what this is. HOWEVER, there are other possibilities:
Some parasites can form cysts and it could be that.
Could just be a benign cyst.
Since this on the top of the fish, you could cut the infected area out, place the fish into a medicated quarantine tank with an antibiotic to prevent any infection until the wound heals over,
You could just quarantine the fish and see if anything comes out of the cyst,
You could take a needle and try to drain the cyst (if it is in fact full of liquid)
Pros and cons of doing nothing:
Pro: if the fish is eating and acting normal, Ignorance is bliss
Con: If this is in fact a parasite cacoon, when it opens, you will have released a parasite into your main tank which could in fact harm your other fish down the line.
This is just another example of the importance of a quarantine tank.