The way I understand it, mushrooms, being fungi, feed primarily on decaying matter. So if one is growing in your pot, something is likely decaying in there. A spore found it's way in and found something to eat. Since it isn't likely to feed on living plant material, it probably isn't a big problem on the whole, but could be an indicator of decay on the inside. If the plant seems fine though, I would just keep an eye on things but leave it be for now. (I'm not an expert on fungi though, so I'm sure some of our mycologists out there could be far more specific!)
Being quite damp on the whole with low light levels in winter, my climate is exceptionally prone to molds and such. The positive side of that means there is moss growing on everything outdoors. But I do tend to be aggressive with molds as they get out of hand easily. I have a household cleaning spray that targets molds (has bleach in it) that I sometimes spray into pots if something is growing. I often use a wooden skewer down the side of the pot to target things better. Hasn't harmed the plants and keeps molds from spreading.