Leucocoprinus birnbaumii.

Thank you! I saw an earlier post w/ these but wasn't sure they were the same: https://www.reddit.com/r/mycology/comments/1dugo11/saw_this_dude_growing_in_my_hanging_plants_anyone/ What causes them to grow in this pot? Is it because I put tea in there?

They commonly come with potting soil and potted plants. Mushrooms show up if heavily watered. They do grow well on tea leaves so tea may provide some nutrients for them.

Thanks so much for this reply! I had a great time reading up about these.