clown knife has blisters, lumps or boils?

i have a clown fish that has grown for a lack of a better word blisters on one side of his head. it looks like the bubbles made when plastic starts to melt under intense heat. there is no red marks, white patches or spots, nothing hanging out of them. he does scrape aginst rocks frequently.

what is this

here are some photo's

http://www.geocities.com/walnut_lover/cn1.jpg

http://www.geocities.com/walnut_lover/cn2.jpg