Green Chromide

I have a pair of green chromides.
The female has what looks like a hole in its forehead and discoloration long the top of its back. This has happened since yesterday.

I will probably use Melafix to try to cure this.
Does anyone have an idea what this is?
Should I use something different?

I took a closer look just now and it looks a lot better than before.
I only things I can think of, is the female might have run into a piece of coral or hit by another fish into the tank or territorial bite from my Dat.
My Dat chases off both my orange chromide and green chromides every once in a while. The chromides graze on the algae on and around the coral head my Dat hangs in.
I put Melafix in the water. The discoloration is gone and there is only a small mark on the forehead.

I have had these green chromides for almost a year. The water conditions are ok. I do a water change once every 2 - 3 weeks.
I clean the tank almost daily.
By this I have rigged a clear hard tube to a soft clear hose on the end of the hose I have a micro screen that catches solids.
I have a sand bottom.
After I am done I clean out the sceen trap and dump the water back into the tank.