There are a number of reasons for Schneeglätte and there is no single term for it:

1. slush
SLUSH
Snow or ice on the ground that has been reduced to a softy watery mixture by rain and/or warm temperatures.

2. snow crust
SNOW CRUST
The crisp, almost icy, surface on fallen snow, usually formed by the slight melting and refreezing of the surface snow.

The least likely:
3. sleet
SLEET
Also known as ice pellets, it is winter precipitation in the form of small bits or pellets of ice that rebound after striking the ground or any other hard surface. It is reported as "PE" in an observation and on the METAR.