I never completely diagnosed this but I'm pretty sure it's some form of mold or algae.
I found that it comes and goes without incident if you're doing regular water changes. I'm doing a 50% WC a week (in a 150 gal). The water literally cures them and it goes away. These spots seem to occur on certain fish more than others. I suspect these would spread if you don't have a weekly water change in place. I'm sure someone on this forum knows the technical name for this.
Langerz - Sorry, I started this thread a year ago and don't have any more pictures. That same fish is still alive and healthy. The fish never acts sick, just gets those spots then they go away on their own.
Miyavp - Sounds like you need to post some pictures in a new thread.