Figured out a main cause of stress. It’s the water from the tap. There’s large amounts of dissolved gasses that have gotten worse thanks to the winter months. So much that it clouds the tank when filling. This would explain the shedding of his slime coat, him not liking water changes, getting darker when the tank is refilling, and the flashing that follows.
The big question, now what?
Smaller, more frequent water changes. I have the same issue here in the winter months, but never had any issues with the FH that I kept in the past.