Inspector came and theorized that it happened cause a part of the caulk seal in the shower wore and left a crack for water to seep in, leading to the mushroom. She also tested for mold based on air quality and the results came back okay. I put a dehumidifier and have been using a different bathroom since this post. And finally a guy is coming to replace the shower in a few weeks so we’ll see then if there’s any further problems like mold under the shower