If mirror cells were released into the environment, they would likely be able to quickly proliferate without much restriction. The natural mechanisms that keep ecosystems in balance, including infection and predation, would not work on mirror life.
Molecules are oriented in a handedness fashion, aka chirality, aka the same things with different geometry. You may remember it from high school. If not, the essential thing is life uses one over the other and a life form based on the geometry not used is the risk and a potential huge risk.