Too much organic matter in potting soil...

Normally I think I strike a pretty decent balance between the amount of organic and inorganic material I use in my potting mix. This time, I think the mix may be a little too organic...

hmmm, yeah, might be a bit too shady too

It actually sits in full sun, and the plant doesn't appear to be at all inconvenienced by its little buddy...

Pity it wasn't a Loph, if it was you could have put it on Ebay as a Peyote and Magic Mushroom combo !

I usually get those bright yellow mushrooms in my pots. Apparently cacti and mushrooms have a symbiotic relationship so I would suggest your cacti are quite happy