I'm gonna say that's a fungus, parasite side affects are normally white nodes on the body this looks like a fairly large grouping which looks inflamed or fuzzy like tlindsey stated. Another give away which is leading me to believe fungus is that it's only affecting your one fish and fungus isn't contagious. Also from the looks of it the majority is only in one major area on the side next to the caudal fin. Chances are it sustained an injury against decor and the fungus took hold from that. Keep the water pristine and start him on an antibiotic triple sulpha.
