A humu will absolutely start a fight sooner or later. They have a reputation of being peaceful until one morning when they wake up and kill everything in the tank.
Most of my fish will fight back I only have two that don't fight a midas blenny that can hide in a gap in my filter and a foxface lo that he would probably avoid by its poisonous spines and if he fights with the octopus there is a very large chance that the octopus will kill it