Final answer:
The expulsion of zooxanthellae due to unfavorable conditions is called bleaching, which can lead to coral death if prolonged.
Step-by-step explanation:
The expulsion of zooxanthellae due to unfavorable conditions is called bleaching. This phenomenon is a result of stress factors such as elevated sea temperatures caused by climate change. When corals bleach, they lose the symbiotic algae that provide them with food, leading to a loss of color and potentially the death of the coral if the conditions persist. The relationship between coral and zooxanthellae is a type of symbiotic relationship where both organisms benefit, known as mutualism.