There are a whole host of soil fungi and with a bit of searching and learning you would be able to narrow down the exact species. I believe they fruit or show mushrooms when conditions are right and probably what you have provided it. Most likely its one of the good guys but always hard to tell. Maybe your watering too much or somthing and that has triggered the mushroom.
I have had very similar ones to you but with a slightly browner cap, quite common and probably not much to worry about except you might want to tweak your conditions so it dosent promote mushrooms but more plant growth.
Wiki soil fungi and see where it takes you