Toadstool Leather Coral

[Jellybeanie]

Got this beauty about a week ago and have been admiring it ever since. From the moment I got it it has been open and large (about 5inches in diameter). Suddenly today all the polyps went in and it closed up. I wasn't too concerned at first until I noticed a few polyps on one side were hanging off. What could be causing this?
I did a water change yesterday (10%) and added some NTLABS Marine trace element replacer (the appropriate dosage) to the water. Could this be the problem? I also added some NTLABS Marine Parazoryne as I noticed white spot on one of my YellowTail Damsels. However this is supposed to be safe with corals.

It's a 55 gallon tank with live rock and sand, jewel protein skimmer, filter and the works, powerheads, etc.

Temp — 27oC
Salinity — 1.023
Ph — 8.2
Nitrite — 0ppm
Amonia — 0ppm
Nitrate — 40ppm
dKH — 10

I know the nitrate levels are high but I have been doing frequent small water changes and it's not changing. Any ideas? I don't think I'm over-feeding.
I have 2 YT Damsels and a Coral Beauty. Could this be the culprit? Or maybe my hermits? I also have a flowerpot that is doing fine. I just got this coral and love it when it's open and don't want to lose it. Is there a possibility it is molting? I read some toadstools do that...
It's usually much more open than it is now...

 

[BigRedsReefs10]

If your doing 10% changes nitrate would only drop to 36ppm. What lighting are you using? The angel may be nipping at the coral(they're known to do that) so watch for that
Edit : do you have rest results for phosphate, calcium, and magnesium?

 

[Jellybeanie]

Thanks for the reply. Would it be better to increase the amount of water changes, the volume of them, or would it be best to do one large one? The coral beauty hasn't had much interest in the coral (as far as I can see) but I noticed a hermit on top of it yesterday morning but it was nearly completely closed. I knocked him off straight away. Please excuse the newbie-ness but I have no idea what any of this means...!
This is the lighting system that came with my tank (JEWEL Lido 200) The man at my LFS said that the lighting provided would be sufficient although he was probably trying to make a sale. Regretfully it was somewhat of an impulse buy but I had been researching the toadstool coral as I read that they were a good beginner coral (starting to doubt that). Thank you for the help

 

[BigRedsReefs10]

Ok so not sure what size tank this is so I don't know if this is realistic for you or not but if you do 3 50% wc's this week your nitrates should fall to ~5ppm. The high nitrates I would guess are part of the problem. It also looks like it's not getting enough light and browning out. Also I missed it the first time I read your post, but you should not have to dose anything to your tank if you are using a reef grade salt, and you should never add anything to the tank that you do not have a test kit for, your weekly wc's will replenish your trace elements. The good news is toadstools are pretty hardy and should pull through if you reduce your nitrates and maybe move it up a little higher so it gets some better light. I'll see if I can find any info on your light