IMO
I think it has been dyed yellow. Up in miami I often see numerous nems that come in yellow like this along with several other types of coral. I was up in miai about a week ago and 3 different fish stores all had yellow gigs as well as yellow sebae's, also several other soft corals that were yellow. When I saw the first gig I took a double look and thought to myself no way cant be a true yellow gig, I proceeded to the next fish store and what do you know another yellow gig, on to the next fish store and what do you know three more yellow gigs. Funny thing is that all the gigs, sebaes, and other soft corals were the exact same yellow. The dead give away in your image is the top of the nem where it has finally started to get some real color back. In my experience when gigs become bleached they usually simply lose all color all together and become white, almost a see through white. I also just remembered that my local fish store here in key largo currently has a yellow gig, I will run up in the morning and snap some pictures. As others have said it could be a bleached green gig and before about a two weeks ago that would have been my first though but upon seeing so many yellow gigs last week and talking with several stores I would lean to a dyed nem at this point. usually I head up to the fish stores in miami once a week, its often funny to see what the color of the week is, all the fish stores I go to get corals from the same whole salers so its jsut a matter of price checking but the dyed stuff always ends up spread out over the 3 different stores and one week it might be purple, next week red so on and so on.