Celestia v1.6.0 draws spurious yellow-green squares around the positions of Nebula DSC objects.
The squares appear whenever any of the four DSC rendering options are selected (Galaxies, Globulars, Open Clusters or Nebulae), although Nebula Meshes are correctly drawn only when the rendering of Nebula objects is enabled.
This problem can be seen by using the following DSC Nebula definition. It causes a square to be drawn around Betelgeuse. The squares are drawn whether or not a cmod model file exists.