Cool! It seems like the conditions became just right for a mycelium to fruit :D
I am not very familiar with the concept of the ecosphere. Is it a closed system, or do you add things over time? How long ago was the last time that you added something into it? I am curious about how long the fungus may have stayed growing in there without fruiting.
Not OP and relatively new to the concept as well, but my understanding is they're typically sealed for long periods (like several months) between brief touch ups. Some people like to seal them entirely to see how long they can last before die off.
Since there are fruits, the oxygen levels were rising and it must have gotten a few degrees cooler in the last 12 hours. Humidity is already going to be perfect in that mini-ecosystem.
While a mushroom is neat, it's a good indicator of what the conditions are like inside that container. A live weather meter, as it were.
It's not completely sealed anymore so there is a small gap. Last "thing" I added were some springtails 2 months ago. Everything else is at least one year in there. I also open it sometimes.
Did expect them to knock on your front door and ask permission?
Parasola plicatilis or the Pleated Inkcap, I think. It should have gills, which I don't see. There are not too many mushrooms that distinct.
If it is an Inkcap, they will desolve quickly, if they haven't already. They are fragile little things..
Cool
