Its black brush algae, its caused by low or fluctuating CO2 levels, and you should identify the cause before treating other wise it may come straight back. Do you water change with lights on, have a CO2 system or run an air pump?
Well if what you say is true then I dont know how you have got it unless our flow isn't too good, clearing it in a low tech tank can be done by doing no water changes for ashorttime but I suggest spraying it with liquid carbon mixed with water 1:20 ratio which will kill it.
The plants you spray should be fine so long as you don't have Vallis and you don't leave it on them for ages,just a quick spray on the algae then refill.