Hi Lee, Welcome to fishlore!

It sounds to me that your
GBR is just shy of his new surroundings and new tank mates. As long as you have plenty of hiding places for him I would leave him in the 55. He will get acclimated in time. For now he is just being cautious.
I think that moving him back and forth to different tanks will be more stressful on it. I would not use any salt either at this point as it just irritates their skin into producing more slim coat which is a protector. If he doesnt need it, I wouldnt use it.
You mentioned that your aquarium readings were all zero. Are you using test stripes or tube for water tests? You should be showing some nitrates if your tank is cycled.
Also you should be careful about the difference in water temps he has been in. One tank at 80*and the other at 72* can be a shock for him. Changes in temperatures should be done gradually.