It sounds as though you have a slime mold growing on your wood mulch. I'm guessing that the lawn and new plants have been heavily watered (probably to help the sod), because this slime mold generally develops when moisture is plentiful. There's not much you need to do except cut back on the moisture a bit.
Luckily, the fungus won't harm anything, and it will disappear on its own when the weather warms up and dries out. If it continues to appear as the weather gets drier and you have cut back on watering, it could be a sign that the area drains poorly.Keep that in mind, and water less often in those spots. In the meantime, you can break up the foamy yellow piles with a garden rake to make them less unsightly. Newly ground or chipped wood mulch is more likely to develop slime molds. It's best to allow the wood mulch to sit for a year or so before it's spread.