Anything that is meant to stay moist (breads, muffins, etc) is subject to mold. Even IP recipe stuff. IP Muffins should be refrigerated and stored in a sealed container with as little air as possible in the container, so that the air won't leach out the moisture. The best way to store, I've found, is in a Zip-Lock baggie, and right before you seal the zip, use a STRAW to suck out the air so the baggie clings tight to the muffins. If the muffins are going to be in a plastic container through the day until lunch, you may want to try throwing in a couple of celery sticks to keep the container filled with moisture, but still keep the container in the fridge.