My clown had the exact same thing. I thought it was lympho also. It only got worse after I let him be for 2 or 3 weeks. It started to bubble out(literal bubble) and spread. I was told to dip him in a hydroplex and water dip. After the first treatment, 80% of the white stuff fell off. After two more treatments, it seems to be on the way out. Throughout all of this, the clown was eating and swimming normally.
I suggest trying hydroplex. Follow the directions of the bottle and see what happens.