as stated, some corals have "sweeper" tentacles that can reach 4" or more. they can wipe out everything around them. these are not to be confused with the tentacles they put out at night to catch food.
some, like many leathers can release chemicals that can kill corals down current (in an aquarium, this could mean the entire tank).
some attempt to digest any corals that touch them.
and some just outgrow corals, covering them and shielding them from light, until they shrivel up and disappear.
as for corals eating fish, a sick/dead fish maybe. these rhizotrochus typus are the closest thing to an anemone IMO
, but they are better because they have a calcium based skeleton and cannot wander!
they have the tools to devour a small fish. you can see by the zoas in the background the sheer size of them-