Could be Lymph, could be a tumor, could be an infection. Not super clear photos to tell, looks like there are some more white spots around the lump?
To be honest, if my fish had lymph I would probably euthanize and sterilize. It won't be fatal disease most likely, but it is almost impossible to get rid of. Causes these type of deformities, not something you want in your prized adult fish. And woudn't be able to re-home anything from this tank without spreading it.