A member of the Sunset Cliffs Advisory Council said it qualifies as "graffiti," and it is up to the city of San Diego to remove the mermaid.
The parks and recreation department has not responded to requests for comment, KFMB reports.
This is not the first time anonymous rock installations have appeared on the rock just off Sunset Cliffs Boulevard.
According to the Ocean Beach community website OB Rag, an illuminated peace sign appeared on the rock in 2008, but it was damaged and stolen. Other art forms have also appeared on the rock, including a dinosaur statue and a replica of Stonehenge.