That's probably happening because of anti-aliasing. If you disable anti-aliasing, it should go away. Of course, that's probably not what you want.
If this is the case, there are two workarounds:
1. Render without anti-aliasing and apply the AA as a post process in a compositing program. I do this sometimes using After Effects and OLM Smoother. This is preferred when I want to use other OLM plugins which work best when the artwork has no AA applied. Then, as a final process, OLM Smoother does a very decent job of applying AA as a post effect.
2. But when I want to render AA in Moho (which is most of the time,) I'll adapt the artwork to prevent the white lines. The easiest way is to simply add a point to the curve and pull it over the edge. I often have to do this when I've split apart the arms at the elbow but I'm still using the full curves for the entire arm in each upper and lower 'arm' layer. Pulling over the edge at the joint covers up the white line nicely.
2a. An alternative way to do this is to make a separate stroke shape that's slightly wider than the edge in the other layer. As in the above example, this will allow the layer to overlap the edge so the white line disappears.
Sometimes the line is caused by a masking effect. In this case, use the Expand Mask By A Pixel option. Or, you can try assembling the mask layers/groups differently. (Sorry, can't get more specific about this without looking at the setups.)
