The leather coral is still alive but is unhappy. Leather corals are soft corals and when they die they start to fall apart and turn into a smelly mushy pile.
If the corals do well in the nano tank but not the big tank, there is something wrong with the big tank. Probably water quality.
When was the last time you did a water change?
How often do you normally do water changes and how much do you change?
Do you make up the salt water or collect it from the beach? If you make it up, how do you go about making it and how long after you make it do you use it?
What is the salt level?
How long are the lights on for?
Keep an eye on the anemone in the nano tank, it doesn’t look happy.