What might happen to a coral reef organism if it were placed in a kelp forest ecosystem

In a kelp forest, with kelp more than 3 feet high, some corals won't have enough sunlight to produce the sugars and food they get from photosynthesis.
Photosynthesis by the way is a process used by plants and other organisms to convert light energy, normally from the Sun, into chemical energy that can be later released to fuel the organisms' activities.