Photosynthesis is a process that uses the soliare to produce chemical energy (glucose). This means that the algae living in the water require a supply of solar light, and for that it would be necessary that the water is clear so that the solar rays can pass and reach the algae.
In the case where these corals and algae live in a cloudy or muddy water, the sun's rays do not pass very well, and therefore the photosynthesis of these algae is altered.
