Mycology

These are actually called "flowerpot parasols", complex Leucocoprinus brebissonii. They're a harmless saprophyte originally from the tropics, they love orchid bark, a common component of potting soil, which is why they show up in flowerpots so often.

You have honeycomb in your monstera?

The texture is amazing.

Is that dried out honeycomb in the bg?

Not an expert, but is that potted plant a tropical by any chance?

I had something v similar if not the same pop up in a 5yo hot pepper plant. Turns out it was a sign of impending doom for that (and most any tropical) plant.

Check with a real botanist, but I would suggest re-potting it asap.