It may be some type of fungal growth or lymphocystis based on similar descriptions on wetwebmedia. For example
"...white cotton like growth coming right out of the gill on the clownfish's left side ... <Well, Adam- sounds a lot like a fungal disease to me. However, many fungal diseases are actually mis-identified bacteria-caused illnesses..."
or
"Within 24 hours I noticed a white wart/cauliflower growth on the
animal's right side, in the white band area, just behind the gills..."
Try searching in these pages.
http://www.wetwebmedia.com/clnfshdisfaq5.htm
http://www.wetwebmedia.com/clnfshdisfaq4.htm
http://www.wetwebmedia.com/lymphfaqs.htm
http://www.enaca.org/Health/DiseaseL...ocystisDis.pdf
"Signs and pathologic changes
Lymphocystis is a terminal infection of individual cells, and the effect on the host is generally slight. The virus is typically dermatotropic and superficial; the condition is much like that of warts, in that the lesions are macroscopic and occur mostly at the periphery of the vascular system.
...
External Signs
Cauliflower-like lesions on body surface, including mouth region, fin and tail region"