Hope everything has worked out as it has been a while since your initial post. If you are using tap water, you could be introducing a number of different things into your tank. I know a lot of people use tap water with success but I have read too many horror stories about tap water ranging from fish/coral kills to severe algae outbreaks. That is why I stick with RO water. It is a small price to pay for peace of mind.
That said, if it does not turn out to be a bump and only a discoloration, you probably have nothing to worry about. Picassos all have different pattern variations and the discoloration might just be from the fish maturing since you got it. I have 2 B/W clowns that were always solid black and white but recently my male has shown a few specs of orange where black used to be.
Again, I hope all is well now. Picassos are awesome!!!