if u can, reduce the light period or totally off your lights for two/three days (take note if u only have fishes and no LPS), all algae will be gone.
then as usual, if u are hardworking enough, use a toothbrush to brush off the algae from the rocks and do a WC 15%-20%
repeat the cleaning and reduce lighting period over the course of the next few days should see your problem vanishing sooner than you thought