Do you have a picture? Light/neon green spots are usually the sign of a bacterial infection, which can quickly become serious if left untreated. I would suggest getting in touch with a vet who has experience treating amphibians immediately. If you cannot find one locally, you can email Dr. David Frye at dr.frye.vetatmilan@gmail.com with a picture of the problem and he can prescribe medication accordingly.

I also agree that you should quarantine the sick frog in a separate tank or container to prevent spreading the sickness to the other frog.

Don't be surprised, but "brand new" pets tend to have the most problems as their living conditions at the pet shop and prior might be anywhere from inadequate to deplorable.