Rubber like skin on surface of chocolate mousse
The cause is that the mousse dries out on the surface. It builds a skin of dehydrated mousse. It happens with most cremes, not only your yolk foam. 

To prevent it, put plastic wrap on the mousse. Don't span it over the bowl, press it on the surface itself, without leaving air between the mousse and wrap. Your mousse will stay soft. This also works for custards and starch-thickened puddings. 

In general, also cover other things you put in the fridge, or they can get an unpleasant odour.