My .02...
2 possibilities...
I starved my rays for 3 days. Had to ship 1 off today. I get home tonight and my hybrid is taking chunks out of mt Marbles. Could be lack of food. Increase feedings and see if it persists.
OR, your "hybrid" isn't ready yet. If this is the case its not gonna stop. Remove the one getting picked on. How big is the tank? Imagine the repercusions of 3 people trying to breed in a closet. Somebodies gonna end up with mixed feelings at best, but more then likely injury is gonna result. Especially when there's resistance.