My guess is that the wilting Fatsia gets more sun than the others. I would move it to more shade or plant in front of it to shelter it from the worst of the sun like the others.

They don't really get a ton of sun. It's been rainy and overcast almost every day for the past 3 weeks. The stalk looks more brown than green/yellow like the others and it looks sick/diseased but there are no bugs around. I'm pretty devastated about this. Everything else is thriving and growing fast!!!

wonder if its a fungal/viral disease that is causing rot.

That's what I suspect. How do I know if its a disease? Do you think I just keep watering them and hope for the best or should I remove to prevent it from spreading? I'd rather replace one than loose a few but I'd love to nurse it back if possible. I'd hate to lose my first plant : (

i'd lift it into a large pot. check the roots for vine weevil grubs too. place in an area of the garden away from the others. and then wait and see if you can nurse it back.

I agree with seaburngirl. Dig it up, have an inspection and pot it up
How long have they been there? Looks a bit like what they do after a hard frost. I had a right panic the first time i saw mine! Clearly this isnt frost though.
Tlc for him then!