Strikes without constricting/eating are usually stress related and is a defensive strike. Something is probably making him stressed. It could be as simple as the mouse approaching him too fast when he was unprepared, or something more complicated like an environmental factor that is making him stressed (too cold, too hot, too much handling, no comfortable hiding place).

If it happens once you can consider it a fluke and he was probably just caught off-guard. If it happens frequently, try to find whatever it is that is making him uneasy.

Good luck!