How big/old is your Grouper...?
When they are tiny 1"-3" they have a brilliant white color and dark contrasting spots..But as the grow their overall coloration fades and blends whith the white taking on more of a "dirty/dingy" look and the spots less distinct...
Being as they are one of the fastest growing fish I'v ever seen, the brilliant "juivanial" stage doesn't last more than a couple months...
Yes the coloration comes and goes during times of "excitement" (feeding) but never as "clean" as when you first brought him home...
Stress is not "neccessarily" a factor in his coloration, but can be concidering th size of this fish and his habitat needs...
What size is your Panther...How big a tank...??