Houseplant Mushrooms?

Question:
Can you do me the kindness of identifying the attached mushroom? I'm not planning on eating them anyway, but I was curious to find out what they are, in the hopes that might give a clue as to how they arrived in my apartment. I live on the 24th floor of a high-rise in Manhattan, so the spores were probably either carried in by a fly, or they've been dormant in the dirt for a very long time. They appeared suddenly in a 30-year-old Thanksgiving cactus that I only water once every two weeks, so I'm really amazed that the conditions encouraged them.

Answer:
I must admit that I am not as much of a mycologist (one who specializes in fungi, etc.) as a horticulturist but just the same I think I was able to identify your new findings. Believe it or not there is a type of mushroom that is commonly found growing among houseplants. It used to be called Lepiota lutea, but the scientific name has been changed to Leucocoprinus birnbaumii. Either way this fungus is commonly called the yellow houseplant mushroom. The spores were most likely in the soil, or perhaps in new soil if you transplanted your Thanksgiving cactus (Schlumbergera truncata) any time recently. I am not sure how toxic or not these fungi are, but I think it is safe to say that they are not edible. If you choose to remove them you can simply pull them out with your hands without any risk of skin rash or irritation.

If you do a Google search for “Leucocoprinus birnbaumii” you should be able to find some interesting and credible resources online. I found one page from the University of Wisconsin that was very informative. I often mention to those who are searching for plant information that knowing and using the scientific name (of plants, bugs, fungus, etc.) when doing internet searches tends to lead you to more credible sources of information. Obviously in your case the same holds true.