Yep those is mushrooms, not the special kind, but mushrooms in your media is a good thing. It means there's lots of bioactivity, the fungus is actively breaking down the compost and making nutrients available to the plants and other micro-organisms.
Those are deliquiessing mushrooms, I forgot the name. You will see the caps turning to a black jelly.
If you decide to eat them for what ever reason make sure you have not had any ethyl alcohol and don't drink during or the day after. You will get sick.