Hmmm... had the person you got it from kept it themselves for a while, or did they purchase it and give it to you?
It looks like either a Yellow Finger Leather, or possibly a Yellow Colt or Cladellia.
Any of which may have been artificially died yellow, especially if Colt of Cladellia, although Yellow Finger Leathers are pretty rare as well.
If it has been died, that would explain things pretty well as these corals tend to get very unhappy and lose the artificial color pretty rapidly.
Either way, died or not, possibly increasing water flow, bumping up the Alkalinity a bit, and frequent spot feedings of zooplankton and phtyo should help.
Also, if you can find out what kind of lighting it was kept under that will give you an idea if it is being light stressed. If it was under lower lighting, moving the coral lower in the tank for a while would be a good idea.