A Semi Successful Mousse

In the neverending search for healthier desserts, I tried making a mousse with yogurt instead of whipping cream. I was inspired by my frozen yogurt recipe, which tastes wonderful even before it's frozen. I found a CIA recipe for Chocolate Yogurt Mousse, so it seemed like the idea wasn't entirely farfetched, but I wanted to make use of the fresh strawberries I had.
The upshot of all this is that I got a great tasting mousse - on the top of the dish. A bit of egg white separated into the bottom of each dish, which was less of a problem for smaller servings and more of a problem for larger ones. I refrigerated the mousse for several hours before serving it - is this what caused the problem? Should I have frozen it? The other thing that occurs to me is that I should simply have used fewer egg whites. Any suggestions?