!
There are a number of different fungi that grow in the organic matter of potting soil. I suspect this is one. I have a bright yellow one that grows in my Ficus plant. I pull them and spray with fungicide, sprinkle with cinnamon... and they grow back.
They will not harm your plant but if you have pets (my main concern), keep them pulled. Mine smell so mushroomy, I am tempted to eat them myself! I can't imagine what my cat would think (Yumm-Yumm!).
Daisy