I'm seeing some Black Brush Algae mixed in with the red
BBA tends to turn red when dead.
Clean as much algae by hand. Do 50% water change / vacuum substrate lightly (don't want ot stir up to much but still want to be thorough). Use H2O2 to treat those areas ( I would only do 3 or 4 areas in a single day. Next day pick a few more spots). I like to put H2O2 into a spray bottle put the entire bottle in the tank pick a few affected areas and hold the spray nozzle about 1 inch away form target. Give 4- 6 good sprays over the area at close range. Just make sure no creatures are in the spray path. You can even soak equipment rocks and some hardier plants that can be pulled from the tank in a H2O2 bath for 15 or so seconds then rinsed and wiped down some.
or Glutaraldehyde. This can be added at elevated levels to help deal with some algae growth... The idea is to overdose the product 2/3xs the recommended levels for a period of ten to fourteen days. However it comes with cautions regarding invertebrate
But the only way to truly deal with algae long term is to correct that imbalance with light and CO2
mixed with the right ratio of fertilizers