Coral bleaching would leave corals with less nourishment, either killing them outright or making them weaker and more easily killed by pathogens or adverse conditions.
Work Step by Step
Reread the description of the normal role of dinoflagellate symbionts in corals. Then imagine what would occur without that nutrition. For an analogy, imagine a little child who is fed by parents— take away the parents (dinoflagellates) and the child becomes weaker.