The dose necessary will be determined by time, concentration of the H2O2 solution, competitive reactants(other organics) and most importantly your target.
In general in small quantities it is safe to add the H2O2 for about 5 minutes with the filter off to some spots. Stay below 0.045mL/L H2O2 (actual H2O2 not the solution volume) and it should be fine even for shrimps. I have to add that the dose required is highly dependent on how much mass and how much organics are available, ie moss in an aquarium may be unaffected by this conc. while moss in a sterile water vessel will turn brown. Look in my thread Algae..Journal ? for some documentation.
But let's start from the beginning ...
What are you trying to remove ?
This is really bad! Can you take the fish out, hold them in a container, drain the tank, and spray with 3% peroxide from drug store. Wait 15 min, refill the tank and put back the fish. The bba will turn red and die off next day. To prevent return, correct the fundamental and dose 2x recommended excel dosage daily.
There seems to be a great deal of debris on the algae. Consider repositioning your filter output to create better flow distribution and eliminating as much of that debris as possible via substrate vacuuming. Otherwise the algae you kill will just be replaced by new ones. High organics means more h202 or excel will be needed.
Excel works as well but I prefer the clean leftovers from H2O2 (o2 and water), but your choice. If there are no plants in the area remove the water and spray directly on the rocks. If you can remove the rocks, treat it outside or bleach it.
The problematic part is not the bba but the cladophora parts. This algae can survive mild conc of excel or h202 and only dies by the point that moss also dies. Plus, it tends to burry in wood like Fissidens moss, coming back when conditions are good. You have to really insist for this algae to go away