Your water change schedule is great. Did this develop after they arrived? None of your other fish have these symptoms?
These spots are very common in discus and are usually water related. But, since they came from a seller who doesn't specialize in discus, they could have picked something up in their facility before you got them. It could be bacterial or possibly an external protozoa. If they aren't spreading then I doubt parasites.
You can stay with clean water and salt for about a week, and if they don't go away you could try a salt dip
http://forum.simplydiscus.com/showth...-Do-a-Salt-Dip
Or you could try a product like Rid Ich Plus.