take the rock out, scrub it with a toothbrush to get all the algae off, rinse, repeat as needed. Oh and do water changes.
I did that to my friends 120, he had cyano
I cleaned off all the rocks with a toothbrush,
snagged off anything that was floating around in the aquarium with a fish net,
took out any sand that had cyano
emptied out the sump,
cleaned it out,
put all new sw
in the sump (about 10 gallons),
did a huge WC
on main (about 50%),
then put phos-ban in the tankwater and increased the tank flow.