Why is the color above the big reef in Xiamen Beach particularly black, while the color below is light gray?

Generally, some marine life will attach to the reef, and the part soaked in seawater will be soaked in seawater, so that the life can survive and the attachments can have life. On the other hand, the upper part is exposed to the sun most of the time because of the tide, and the attachments are sunburned and oxidized, forming a dark color. These dark things turn black after a long time, and most of them are algae, such as seaweed.