silver mollie keeps nibbling my black moor

She could be pregnant, female fish tend to be more agressive toward male fish when they are pregnant, either that or the fish she is grabbing could be sick, because if another fish is sick in the tank then the other fish will kill them to illiminate any threats. Keep an eye on the male fish that keeps being attacked, if you start seeing any white spots on him then get him out of there ASAP and treat your fishtank with an ick killer.

it is possible she is prego, but consider this: did you just put anything new in the cage that the black moor or the mollie fight over? you know, for dominence? did you happen to touch any fish, while you had food grease or crumbs on your hands? they may be atracted to the smell or taste. make sure your fish dosnt have any desieses either. like fin rot, becuse it will die and usualy after a fish dies it will be eaten by the others, and so if your fish is dieing the fish may want to hurry and kill it so it can eat it.