If that was indeed frost, and not condensation, that aircraft should not have dispatched.
The E145, like many other aircraft, is tolerant to frost on the underside of the wing but not on the upper surface.
The vast majority of a wings lift is generated from the upper surface. Ice, frost and other contaminates disturb the airflow over the wing surface and change its characteristics. This can have a dramatic effect on the stalling profile of the aircraft - and never for the better.