Yellow Gobie is white?

[smallblock]

I have a mated pair of yellow watchmen gobies, I looked at them today and one of them is white. Anybody got any ides on that. I know at night i think they may fade a little like my yellow tang. The tang turns lighter and a white stripe shows up on the side of him. But the gobie came out to eat and no change the other one looks perfectly normal. I have a 55 gallon with a yellow tang (aka hogzilla) a coral beauty, blue damsel, cleaner skrimp, blood red skrimp, emerald crab, a few hermits and turbo snails

 

[drnecropolis]

The white one is a female..

 

[smallblock]

really, have any idea why she does that? most of the time she is just as yellow as the other