Dont believe its aggression. When stars like these lose their legs, it can be bacterial health or poor water quality and even lack of diet . Starfish mainly lose their legs when theyre not getting enough food and in essence starving to death. You should always make sure that there is enough food for your starfish which eat small inverts, shrimp, larger pods and even small urchins. When starving they will bury themselves into the substrate and disintegrate and eventually starve to death.
They also cant tolerate warm temps for lengthy periods above 79 degrees