Very confused

[Afrashz]

Hey guys, so about 3 weeks ago I purchased a little zoanthid colony from my local fish store and it immediately opened up and looked beautiful in my tank. It looked great for the following 3 weeks, until yesterday..... This is when I noticed about half of this colony was closed. I figured maybe my clowns were messing with it or something and decided Id check again today after work and see if they reopened. Unfortunately they did not. Next I did about a 15% water change so hopefully that helps. My parameters are all good for the testing capabilities I have:
salinity- 1.024-1.026
ammonia-0
nitrite-0
nitrate- 10ppm
temp- 78-79
I have no pests that I can see with my naked eye and all I have is 2 clownfish. All my other corals are doing well including parts of this colony and other zoanthid colonies. (knock on wood)

If anyone can give me some advice it would be greatly appreciated. Ill post a picture below. Thanks.

 

[nfrench2100]

Did you dip them before putting them into your tank? Just because you can’t see any pests with the naked eye, doesn’t mean they aren’t there.

If I had to guess, seeing that this is the only coral that is acting this way, and it being the last one placed in the tank. That some sort of pest is likely the culprit.

If it were me, I would mix up a little tank water with my Coral Rx, and dip it immediately.