it IS normal for the Hippo Tangs to suddenly lose thier coloring. i have noticed it MANY times in mine AND at the
LFS that i work for (part time). it is more common when they sleep, but does happen from time to time when they are not asleep.
you may want to keep an eye out for some type of stress factor in the system, and keep his diet varied. this could be due to lack of algea or meaty foods also. or another fish "picking" on him.
HTH