Mapping Center Answer:

This effect is also known as the "halo".

The problem is due to partially transparent textures within the symbol, where the background color kind of “blasts through” surrounding features to add a glow or halo around the symbol.  The good news is that the effect can be reduced by adjusting the "Minimum transparency threshold" slider on the Rendering tab of the Layer Properties dialog.  Zoom to a viewing position within the document in which you see the effect, then adjust the slider until you get the desired mix of transparency and lack of halo.