it's always like that, it only makes one and it's not enough for risotto😂 jokes aside... mushrooms form if there are spores in the soil, even brought by the wind or on some sleeve of the shirt these spores need to find a suitable environment to grow and reproduce when they find the right environment, lots of humidity and soil that is always wet or damp, the mushroom or mushrooms form to avoid this, just let the soil dry well between waterings of course, if you let it grow, the risotto won't turn out the same, but it can spread other spores remove the first few cm of soil from the pot and replace it with new soil