I just had my enlarger apart, and used the little bottle of eyeglass cleaner that they gave me w/ my new glasses. Worked a treat. It's probably mostly alcohol. Plain old dish soap and warm water also works well.
My Philips PCS150 has a mirror just before the condensers and it is not front surface, but I have never had a Durst with the condenser head. As Matt points out, make sure it is not a front surface mirror before trying to clean it.
Since the mirror is not in the optical path but the light source path, it probably is not a front surface mirror, but it is easy to distinguish between the two by looking at the cut edge of the mirror.
Since the mirror is not in the optical path but the light source path, it probably is not a front surface mirror, but it is easy to distinguish between the two by looking at the cut edge of the mirror.