It's the water soluble proteins being released from the chicken as it cooks. They mix with the fats being rendered from the skin and with the water the chicken is being cooked in and that mixture floats. It's perfectly safe. The foam may even turn brown or grayish as the chicken cooks. And as a matter of fact, it happens with any meat you boil at a high temperature. For the purposes of our chicken broth recipe, it will all be gone when the broth is strained at the end.