Grayed notes are notes that 'belong' to a previous note, i.e. the first inserted note of a chord is displayed in black, all others in gray. Grayed notes are not taken into account when determining bar duration.
Grayed rests usually are rests that are appended automatically when MusiCAD makes all parts of equal duration and will be removed as soon they become obsolete.
It is possible to make most objects invisible on the main screen using <x>. Thus hidden objects will be displayed in gray during edit. Rests can be hidden this way too (useful when assigning more than one part to a staff).