it's prolly just spores that are in the mix... It wont harm anything.. but it can be an indication of too much moisture, so take it light on the watering.

Frank, I agree with M3..Most likely, the soil is too wet. Is your AV getting adequate sunlight/light?
Mushrooms won't hurt a plant, but when one pops up it indicates soil needs amending, less water and/or more light.
If the AV was mine, I'd remove plant from soil, discard (soil), either use a new pot or clean the older pot thoroughly, and repot in a well-draining mix.
Good luck, Toni

i just repotted it yesterday, which is the weird part. I did just water it afterwards pretty thoroughly, so I'm sure that's part of the reason. It's off to dry for the next few days or, or however long until I water it again. I just thought it was odd how fast they sprouted. It's a few feet away from a southwest facing window, where I have the rest of my cuttings rooting.