Do you have a photo of it that can help us see it and better diagnose it? Does it look fluffy or smooth?
Could he have injured the eye on a piece of sharp decor or got into a scuffle with another fish? Even peaceful species tend to spar with one another from time to time.
What are your usual and current readings for ammonia, nitrite and nitrates before your scheduled water changes? If you have any presence of ammonia or nitrite, or your nitrates are elevated (more than 20 above that for your tap water level), it's possible that water quality is the source of the problem - whether short-term or long-term impaired water quality can affect the fish's immune system and cause it to develop a health problem, so it's important that that aspect is considered.
What type of fish is he, what tankmates does he have, what is your tank size, and is your water soft or hard? Answers to these questions will also help us work out whether there's a potential underlying reason for this growth.
The good news is that the fish is eating well and behaving as normal but, once we know the answers to the above questions, we can provide further advice and whether or not it's worth medicating him. More frequent and larger water changes certainly would be advisable in the interim if that's possible, to optimise water quality.