That looks like and is typical for that planted discus tank: staghorn algae, Compsopogon.
Excel/Easy Carbon will kill it pretty good.
I'd like turn the filoters off and then pour the Easy Carb around the infected areas and leave the flow off for 5 min.
H202 3% spot dosing the same way can be done, do not add more than say 1 ml per 40 liters of tank in any 2-4 hour period.
In organically overloaded planted tanks, classic discus tanks with plants..this stuff is very common.
Tanks that lack frequent water changes tend to get it or have high fish loads, over feeding etc and poor CO2.
Some combo thereof.
Better CO2 management, better general care of the tank, more water changes, filter cleaning, dosing routine etc.