This may be too late of a post but hopefully the toby is still doing ok.
Mysis shrimp would be better food. Soaking it something like Selcon would be even better.
First thing is test the water for temp and specific gravity. Ask the owner what the tank usually runs at and see if there has been a swing in those parameters. I would then test the water for ammonia, trites and trates before you do anything else. Do a WC if needed.
Hestiant to recommend setting up a hospital tank since you haven't had SW in the past. You could use the existing tank water and replace the water in the existing tank. To do so, all you need to do is match temp and specific gravity of the new water you mix. This should limit the impact to toby since you will be using the same water and the current display should do fine with a WC.
You will need to oxygenate the water somehow (pump, PH, HOB filter with decent tornover, etc). Be prepared with Amquel (for ammonia) and mixed SW ready to go for frequent water changes. Don't overfeed the QT. Only give what the fish will eat (if it will).
Even more hesitant to suggest meds but if the toby is new, it may be flukes or other parasites (ich) in which case Malachite green, Cupramine, Clout or PraziPro may be an option. The clouded eye and clamped gill suggest flukes might be the cause but if the fish has been in the tank for a while without sysmptoms, it shouldn't be that.
If you think it is bacterial Maracyn Plus may be an option.
If you choose to medicate, DO NOT medicate the display tank.
Good luck.