Corrals contain a huge mix of live together with the symbiodinium called " the coral holobiont" Most nutrients used by the coral are organic and retrieved from the holobiont and the surrounding water. Corals are able to manage the growth rate of surrounding life and its holobiont by releasing mucus, containing organic carbon and phosphate. Such mucus may contain 200x the phosphate present in the environment. . ref: MB Anthias 2019There was a video on YouTube from Reefdudes speaking with Lou Ekus from Tropic Marin. He explains that the corals are not good at taking up phosphate from the water column directly. However beneficial bacteria could take up the phosphate and then the coral would get phosphate from ingesting free floating bacteria. If that is accurate then possibly even if 95% of bacteria is on surfaces the corals are competing with the 5% that is free floating with a skimmer to get phosphate. I don't know if this accurate but seems to make sense.