This.
The squirrelfish and eel are both crustacean eaters.
Reef tanks thrive in captivity usually based on making sure you have the appropriate number of snails and other misc crustaceans in there to maintain the balance in your aquarium. Without them you risk the growth of cyano bacteria and hair algae. Both are known to "choke" corals.
Plus if they like the taste of a coral they just might take a bite out of it.