I need to find out the adaptations of algae and seagrasses so they can survive in the coral reefs

Answer 1

I have never seen healthy coral coexisting with seagrass or algae.

According to a website summarizing a recent scientific meeting on dying Caribbean coral, summarized at
(Broken Link Removed)

"Unlike corals, seagrasses and mangroves, Caribbean algae appear to be surviving well and perhaps are taking advantage of the corals' demise. Algae thrive on dead or dying coral reefs and can overgrow and smother newly settled corals."