Male goldfish get a white bump on their gill covers but I'm assuming that's not what it is, right?
There's a good chance it's a bacterial infection. Rather then medicating right away I would first try to find out the source of the problem if there is one. Get the water tested for ammonia, nitrites, and nitrates. Do a water change, add some aquarium salt
(that will help if this is actually being caused by parasites), and quarantine the fish if possible (since you don't want it spreading to the other guys). How often do you do water changes and how much do you change? Do you vacuum the gravel too? Since goldfish are extremely messy it's important that regular water changes are done and you'll want to have excellent filtration as well.