I regularly encounter fellow cichlidophiles who are perplexed when it comes to cleaning algae off the rocks in their tanks, including holey rock. There are many different ways to rid your rocks of algae buildup, some more labor intensive than others. If you’re not in a hurry, the easiest and most tank friendly way (no chemicals used) is to simply sun bleach them. What does that mean? Just put them in the sun for a few days. Afterward, they’ll look as good as the day you got them. How many days depends on the volume and type of algae covering the rock.
The photos below show two holey rocks covered in green algae and the same rocks after three days in the sun.