It's called peppering which is quite common in pigeon blood strains or any strain carrying the genetics of the pigeon blood (PB for brevity). It will be there for life although it can be minimized by putting in a tank with light background or doing plenty of water changes to keep up with the clean water quality. Peppering is not necessarily a bad thing for aesthetic reasons but it tends to show up more when the specimen become stressed although as I said earlier, it can be avoided. Sometimes the peppering diminishes when the discus appears to improve in its health condition or to clarify matters, not stressed.