If you are using gelatin to set the mousse, do not fold the gelatin mixture into the cream until it is cool, as this may cause the mousse to separate. The thickened mixture should be of a similar consistency to the cream before the two are stirred together. Stir the mousse mixture continuously while combining the gelatin mixture with the cream so it does not sink to the bottom, giving a layered effect.
If the mousse is being made with whisked egg white instead of, or as well as, gelatin, fold it in gently so as not to lose the air bubbles which hold up the mousse. If a mousse does separate, there is no remedy. However, it should be perfectly edible, even if it is less attractive then intended.
** Dessert Recipes