Can't say as I have any experience with a mushroom popping up in the soil media, but if you are concerned, you can always remove it. It sounds like a stray spore found its way there. I ususally get stray sundews or U. subulata or univited carpet moss in my CP containers!

Out of curiosity, are your plants in a terrarium or are they exposed to the air? Inside or outside?

I wouldn't be too concerned about root rot. Botrytis, Pythium and Phytophthora (the most common root pathogens you'll encounter) don't produce large fruiting bodies. Sounds like there's something in your potting mix that is deteriorating. It may be the sphagnum but I'd be more inclined to think there's a bit of organic debris (eg sticks, leaves) in the sphagnum that is breaking down. Lower grades of sphagnum often contain sticks etc. The fact that the mushroom is growing where the plant gets watered probably has more to do with moisture than anything. FWIW removing the mushroom will do little to fix the problem (not that there is necessarily a problem). There's still mycelium throughout the mix. You can either live with the occassional mushroom or replace the mix (maybe not a bad idea after two years). In this instance, fungicides are unnecessary and may do more harm than good.