Lymphocystis is actually becoming a more common finding in the domestic goldfish/koi world, mostly due to the ‘silent carrier’ nature of asymptomatic fish. The virus often infects these fish then lies latent, sometimes causing no disease in the carrier fish. Still we tend to see a problem with this virus when it does recrudesce and causes these viral induced masses to appear in the infected fish. This recrudesce is often linked to a weakness in their immune system (either because of their age, stress, if the fish is immunosuppressed or fighting some kind of infection that takes the attention of the immune system). And it is this time of weakness that triggers the herpes virus causes its mischief.
I am afraid that there is no treatment for this virus, and with time they tend to increase in size and can become pedunculated and prone to trauma/infection. We can sometimes surgically remove them, but the stress of surgery can potentially lead to more lumps appearing.
With all this in mind, I have to warn you that the only reliable way to get rid of this virus in your pond would be to cull all fish that are in this pond (which would be unfortunate) and have a full drain/sterilize (probably wait a few weeks) and then start again. That said, I personally wouldn't advocate for this but instead would advise that it'd be kindest to maintain the stock you have, know this is a risk and make sure water parameters are as best they can be and disease risk factors are as low as possible, to deter lymphocystis recrudensece. I would say if you chose to continue with this stock (I probably would because it is a benign condition that they can still live normal lives with) that you should perhaps avoid bringing in new fish since any new comers could contract this (if they don't have it already) and develop masses as well.
Please take care,