I had a Yellow Watchman Goby that turned gray and never colored up again. Very happy fish, just lost its color and never got it back? Reeferrob has had it in his grow out tank for almost a year with no changes in color?
I have a Tailspot also, they have stripes when they sleep. They'll turn white out of fear also. I had mine do this once when I was transferring him to a different tank. The Ecsenius genus is known as being color changers.