Sorry for being a little bit rushed in my previous post. There's a couple things that you can do. You can either take it back somewhere for a trade, or you can try to nurse it back to health.
Goniopora come from extremely nutrient rich waters, and although they are photosynthetic, they rely heavily on constant feedings to survive. For this reason, they are usually on the way out by the time they make it to the consumer due to starvation. Goniopora are also extremely susceptible to contracting a disease called Brown Jelly. No one is exactly sure why they get this so often, or really positive on what Brown Jelly is, but if the starvation doesn't kill the coral, the Brown Jelly will.
If you want to try to nurse it back to health, you need to go out and get an Iodine dip for Corals. This can either be Seachem's Reef Dip, Kent Marine has one too, or Lugols. You need to prepare a bucket of saltwater (with the proper pH and temperature) and do a double dose of Iodine, and leave it in there for 10-20 minutes depending on how it reacts to the dip. This may clear up the jelly it may not. If the coral was starving to begin with, it'll probably die anyways. You need to dip it twice a day for several days, all the while watching closely.
If the infection is going to get better, you'll know pretty quickly. Don't be surprised if it loses a bunch of tissue. If it isn't going to get better, you'll know because the Brown Jelly will not clear up.
I strongly recommend you visit goniopora.org and speak with John Kelley, or please find someone that can properly take care of this coral. They need a lot of TLC to be happy.