In rocky desertification areas, moss pioneers on karst slopes where rock-surface flow dominates; how it interacts with micro-topography (inclined plane, concave, convex) to jointly regulate runoff, hydraulics, and erosion remains unclear. Using rainfall simulation experiments, this study systematically examines how rock surface morphologies and moss cover influence runoff coefficients, sediment yield, and hydrodynamic parameters, to elucidate the erosion mechanisms driven by rock surface runoff. The results show that sloping rock surfaces generate the most runoff, with runoff coefficients 16.5% and 5.3% higher than those of concave and convex faces, respectively. Convex faces suffer the most significant erosion, losing 33.95% and 50.25% more sediment than concave and sloping faces. Moss significantly reduces the initial runoff coefficient for all morphologies and narrows the flow velocity differences by 89.7%. It also removes the concave face's low-erosion advantage, making the convex-vertical orientation the most eroded (an increase of 28.8-53.8%), and thus becomes the primary driver of slope erosion. A direct link exists between rock surface and soil-surface hydrodynamics; runoff generated on different rock morphologies indirectly affects bedrock slope erosion by altering the distribution of slope flow, resulting in either concentrated wash or dispersion. Results demonstrate how the moss-biofilm cover on different rock morphologies affects karst hydrology and erosion, providing key insights for targeted conservation and sustainable land management.