Plate Coral not happy??

[365seasons]

We recently had to transfer everything in our main 65G over to our 55G to have the overflow repaired. The transfer over went fine, and everything has been in there just over a week.But our plate coral is not doing well at all right now.

It is squished in the tank, as it is a column, and with the rock, it leaves little space on the bottom for our big coral, and there is no sand in there. This was just supposed to be a temporary (few day thing) but is ending up taking longer.
The plate is not coming out, and is covered in a brown-ish slime.

Is it just unhappy, or is it dying??
We are going to try and transfer everything back over to its original tank tonight, but it will have 65% new salt mix, and only 45% of it's original water. Is this a good idea??

Any ideas on what is happening or if there is a way to fix it??

Thanks in advance.