Strange Foam

I noticed the other week that there is this strange stuff in the garden. It has only popped up in the flower bed with the day lillies. It's so weird because it'll show up one day, almost growing for a day or two and then wash away with rain or just disappear. The stuff is airy and light, kinda watery, I think it may be some sort of mold. The mulch we have that the builder put in is not high quality, so that may have something to do with it, I think. I want it to go away forever, it looks gross.