10 circle marks in yellow tang overnight?

[CBehr]

I just noticed about 10-15 circle marking on my yellow tang last night that I didn't notice the day before. Looks like the yellow is gone.... what could this have been? I can't imagine it's any type of disease as I haven't added a fish to the tank in probably 6 months. I think I remember hearing stray current could do this...but everything is on GFCI's and I've had my hands in the tank numerous times yesterday/today.