Coral bleaching occurs when a colony of polyps are under stress.
a. true
b. false

Coral bleaching indeed occurs when a colony of polyps is under stress. 
Coral bleaching is the loss of endosymbiont's intracellular due to the explosion of the pigmentation of the algae. When the colony of polyps is under stress, it causes the polyps to expel the zooxanthellae from the coral.

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Although i don't know what is the simulated system, starfish are predators that eat mostly  microalgae,  snails, sponges, bivalves, mussels and other small marine animals. removing starfish from any system would definitely increase this animals that they prey.