G'day Alex. Those white dots are discus pimples and happen as a result of poor water quality. They can progress to hole in the head if left unchecked.
The very dark wild cross discus isn't looking good. How long has it been dark like that? Is it eating like the rest?
Are your nitrates 10 before or after your water changes? Whatever the answer, increasing the size or frequency is the answer to maintain nitrates <10 on your water change day. I would add 3-5tbsp/10gal salt as well and replace with water changes for 10-14 days, then reduce back to no salt gradually. I recommend you start aging your water before water changes. Micro bubbles from pressurized water service lines can/will make your fish stressed and prone to sickness. Heat and aerated the volume you want to change in a food grade barrel for 24 hrs before your water changes. Add prime as you transfer the aged water into your tank. This will also make a huge difference.
I would not recommend adding any more discus until this current situation is a thing of the past.
Keep us updated on your progress