I am sill very new to this but it could be:
Lymphocystis (Lymph, viral): Lymphocystis, Viral Disease
White to gray clumpy, cauliflower-like appearing bunches most often on unpaired fin membranes. Expresses itself most often in stressed individuals, poor environments.
Pulling off large/r masses seems to trigger the host immune system. Improved water quality, nutrition. Possibly the addition of a purposeful cleaner organism.
Here is a good website on clownfish diseases: