Protection
In the parrot fish family it is quite common, when they go to sleep for them to enclose themselves in a cocoon of mucus, to prevent predators from smelling them out, because at night vision is useless and most predators at this time will purely hunt using smell and touch. You would think the Humphead parrot fish doesn't use this method when it gets large enough, but they still are hunted by large sharks so at the end of the day the school breaks up and they retreat to their own little crevasse. During the day because they live in large schools, they are well protected from the average predator looking for a meal.
Symbiotic Relationship
While the Humphead parrot fish doesn't really have a strong symbiotic relationship with animal it does share a symbiotic relationship with the cleaner wrasse (who shares a symbiotic relationship eith most animals in the sea), who cleans out all the bugs and oceanic parasites growing on the Humphead parrot fish as it can not clean them off itself. So the Parrot fish will approach a cleaning station where a cleaner wrasse has set up shop and will remain still while the parrot fish gets cleaned and the cleaner wrasse gets an easy meal.