You already told them that the program is accredited, and you're able to take the NCLEX, and that's basically what your employer need to know. As far as their ignorance of distance learning programs, you can't change their opinions. Excelsior is a real school that is located in New York which offers distance learning programs, just like thousands of universities in the U.S.
My coworkers were very understandable when they knew I would be testing out of classes and clinicals, since they told me I was pretty much working on an RN level anyway, but getting paid as an LPN.
If your coworkers have any respect for you, which it seems like they don't, they will recognize your skills and knowledge, and have confidence that you're able to do distance learning without having to sit in a classroom and starting with the basics-->skills you're probably performing everyday at work.
If you already informed them about Excelsior and they still don't approve of it, just ignore them. Go to work with your head held high, performing your skills at your best! If they keep bugging you about Excelsior, ask them if they're willing to pay your bills while you're enrolled full-time to a "real college" getting a "real education"
Good luck with your studies!