Hi @ryanfleming,
unfortunately it looks very much like lymphocystis.
Quarantine the fish immediately to avoid spreading of the virus to the other fish.
The virus can be inactivated with a bath of potassium permanganate (100 mg/L or higher), or formalin (2000 mg/L or higher) for 15 minutes at 77°F (25°C).
These are quite high doses. So be sure to use proper precautions (adequate ventilation and personal protective equipment, e.g., gloves, goggles, when using these chemical meds.
The problem is, as you can see, those are such high doses that it is almost impractical.
If you want to try treating it, get some formalin
https://www.thetechden.com.au/Aquasonic_Formalin_Malachite_250ml_White_Spot_p/pl124.htm
Drip a few drops directly onto the growth. And 1ml into 1L of water with the fish. Keep aeration going as formalin will deplete oxygen in the water. Move the fish to clean water after 30 minutes in the formalin bath.
I would also use it to treat the main tank at 1ml to 40L.
What are your other water parameters? pH, temps, GH, and TDS if you have a TDS meter.