first cover the medium and the tanks exclude all light possible.
drain nuits, flush well, then run a dilute H2O solution without air pump,throw away your air stones or soak in H20 full strength after a couple days, drain, flush and start back on the nuits with air.
Alge doesn't really hurt it just out of balances your nuits,it eats a lot of N. it needs light,food,O2 to grow

Daddy