what is it called when Corals feed off the byproducts of a microscopic algae living within their own tissue, called zooxanthellae. The photosynthetic activity of the algae is vital to the survival of the coral animals, which use the energy to extract calcium from the seawater and build their calcareous skeletons. The zooxanthellae are protected by the hard coral and obtain plant nutrients from the coral

The correct answer is - mutualism.

Explanation:

Like it is given here, The coral gives a protected environment for algae and other compounds algae required for photosynthesis. Photosynthesis performed by the algae provides a byproduct of microscopic algae.

These algae produce oxygen and help the coral to remove wastes and algae is important to the survival of the coral animals, which use the energy to extract calcium from the seawater and build their calcareous skeletons.