Juvi female yellow goby?
 

Has anyone actually seen a yellow watchman goby turn grey? I think mine is as it's a dark yellow In the morning and has these grey spots growing on it's back. Still eating and moving and I can't find anything about a disease with growing grey spots nor can I find pics of a goby turning grey

Here's some info,

One thing I have omitted so far is pairing the gobies. In what I have seen of this fish, I believe they are sexually dichromatic, meaning that the male and female are different in color. With every pair that I've seen, the male is bright yellow while the female is grayish, with both having blue dots along their body. But what makes this confusing is that I have seen this fish change color in both directions. Does this mean that they are changing gender as well? I do not know. After the babies settle out and metamorphose, they are yellow. Then, as they grow over the next few months, most of the largest ones will change to the female coloration. From a few people who we have sold these fish to, we have reports that some changed back to yellow after they received them. Despite this confusion over coloration and gender, it is not difficult to form a breeding pair. I believe the best and easiest way is to pair one that is larger and gray with one that is smaller and yellow.

http://reefkeeping.com/issues/2005-12/ft/index.php