First step would be to remove the affected rocks, etc. and give it a good scrub with diluted bleach. Rinse until the slickness of the bleach is gone then soak in water with declorinator before putting them back into the tank, algae side down.
Once that is done (you were removing spores and decreasing the amount of algae to fight) make sure all nutrients are available (N,P,K, CO2, trace and light) and the algae will die off.
Likely an excess of PO4 is the cause of the algae you're experiencing BTW.