I probably didn't give enpugh context here, but the maincoral gets nutrition from the colourful algae on its surface. When temperatures rise, the algae gives off toxins, so the main coral rejects the algae (and then later dies because it has no food source).

It's really hard for that kind of process to evolve out, but some scientists are having success hardening coral in labs (so it'll reject algae less hastily) before releasing them into the wild