As others have pointed out, that SPS was most likely not very healthy when you purchased it since it appears the base has been dead for a while. Combine that with some stress and/or less than ideal conditions and it can easily lead to the complete demise of a colony.
Just some random thoughts...
I'm not sure how long that tank has been set up but just from the pics you've posted it looks fairly new. New setups and SPS don't mix well and many people suggest not adding SPS during the first 6-12 months after an aquarium is set up. I think this is especially important if you're fairly new to the hobby. That goes doubly for a smaller aquarium where parameters can get out of hand a little more quickly.
If you did not slowly acclimate the new coral to your lighting that could be another factor. All corals should be started at the bottom and slowly moved up over the course of 2-4 weeks. Just judging from the appearance of that coral it doesn't look like it was subjected to direct bright light prior to you purchasing it which makes acclimating it to your lighting even more crucial.
You didn't mention your alkalinity or magnesium levels. Alkalinity/dKH is probably the most important thing to test for and keep at proper levels in a reef aquarium. It's also about the easiest thing to test if you use the cheap API and Tetra test kits. They're actually pretty accurate test kits and I'd suggest them, especially with the recent issues with the Salifert kit.
Just because the things you test for seem in line does not mean something isn't wrong with your water. There are so many things that can be wrong with our water that we can't test for. Heavy metals, high levels of DOC's, imbalances in various elements that make up seawater, and elevated levels of various chemicals released by corals and algaes, are a few that come to mind. I only say this because the other two zoanthids in your photos don't seem to be doing particularly well.
On a related note... What type of skimmer are you using? Do you run activated carbon or any other type of chemical filtration? How often do you do water changes and what %? What brand of salt mix do you use? Are you using RO/DI water for makeup and do you know the output water is 0 TDS? What types of supplements/chemicals have you used in your aquarium? Have you tested your PH in the morning before lights on?
On the flipside, I don't want you to automatically freak out, your water could be great. Sometimes corals just die for unknown reasons, especially when they're not totally healthy to begin with. In addition, maricultured SPS (which is what yours was) isn't going to adapt as well to your auqarium or be as hardy as a tank grown frag. Because they're small colonies they are quite adapted to the environment in which they were grow. They grew the way they did based on flow, light levels, available food sources, etc. etc. For that reason I'd suggest you stick with frags from here on out. At the very least just make sure, if you're going to buy a colony, that there are no bare spots or pests and that it is has been around and healthy for a little while at whatever LFS you purchase from.
Pests are a possibility, but my gut tells me they weren't a factor. Even if it was pests they will rarely lead a quick death like that. However, it's a possibility since the coral was probably not fully healthy to begin with.
One last thing... I notice what appear to be dinoflaggelates (the slimey brown stuff with air bubbles) on the base of the coral in question. Is this something you've noticed elsewhere in the tank?
Sorry for the wordy response and all the questions. Just trying to help as best I can...
Peter
BTW-All my params seem to be good at this moment. Here's a few pics 
