2 EEs were assigned the task of hanging a solid curtain in the x-cut between No. 5 & 6 entries. EE was working off a 6 ft step ladder & was descending ladder when he slipped to the mine floor as he was stepping from the 3rd rung of the ladder to the 2nd rung. He complained of pain in back but finished his shift. The area was wet & muddy and there was mud on the ladder rungs.