"mach" is the imperative form of "machen" and perfectly fine in that sentence in the sense of "Honecker, please turn of the lights!". With "macht" the sentence

"mach" is the imperative form of "machen" and perfectly fine in that sentence in the sense of "Honecker, please turn of the lights!". With "macht" the sentence would be more of an observation and the "bitte" (please) would sound weird: "Honecker please turns of the light." Edit: as tazjin stated "macht" can also be used to address the whole group.