What is my duncan doing?

[TNix]

2month old tank
Duncan has looked great for 1 month (growing polyps)
1 week ago 2 of the 4 polyps closed
Phosphates were a little high so i got them down with less feeding over a week
Then i realized a metal thermometer was left in my AIO overflow. Had some rust on it. So took it out and a few days later the 2 polyps opened. Not sure if it was the phos or iron? Then, few days later same 2 polyps’ oral disk enlarged twice the size (see pic)
All other parameters ok
I am very new to reefing, any ideas?

 

[MERKEY]

What have you been using to remove the metal that was leached into your tank from the rust?

There is cupisorb, metazorb, polyfilter...

 

[TNix]

What have you been using to remove the metal that was leached into your tank from the rust?

There is cupisorb, metazorb, polyfilter...

So I didnt add any chemicals. I am trying to avoid. I removed the thermometer (it had a very small area of rust). I changed out my carbon because it was due anyway and I did about a 20% water change. It is really the only thing that is closed up. Not sure what is going on, this Duncan looked so good for about a month. I have another duncan that is doing great and looking good.