Firefish! Nemateleotris magnifica

[Beano1169]

Hey all I have some questions my firefish in my tank has developed a blackish/greyish splotch on the top of his head. Its and odd discolouration and I was just curious if anyone has dealt with something similiar? Any suggestions would be greatly appreciated but other than that he has been eating like crazy swimming around lots and enjoying his time in the tank very much so!