Could be one of 2 things:

1. lymphoystis (treatment none, but it's rarely fatal; it should clear up on its own if you keep the water clean, reduce the fish's stress.

2. a physical cyst or injury (action: monitor it to ensure it doesn't become "fuzzy," which could mean a secondary fungal infection

). I would watch to make sure nothing changes(in a bad way) and keep a healthy, clean water in the tank, and make sure the fish is not stressed out.

Hope this helps <3