If you're using this one you have the method
public void setBalloonBottomOffset(int pixels)
Set the horizontal distance between the marker and the bottom of the information balloon. The default is 0 which works well for center bounded markers. If your marker is center-bottom bounded, call this before adding overlay items to ensure the balloon hovers exactly above the marker.
So, if you have a marker that has the usual V at the bottom, defining the point where it touches the map, you need to set an offset of half it's height BEFORE you add it to the overlays.
Example:
----- ^
| | |
| | 20 px of height, you need to do marker.setBalloonBottomOffset(-10)
-- -- |
V v