Just asked my botanist spouse (hobbyist but quite experienced and well-read in all growing things), and he thinks it's not a good sign; mushrooms aren't supposed to grow in potted plants _at all_. Reason might be too humid/too much watering, and repotting it with removing as much of the old soil as possible wouldn't be a bad idea.
Additional question: is there any chance that a domesticated critter has mistaken this as a litterbox? We're always getting mushrooms of such kinds in the most untypical spaces anywhere a cat has sh**t in the garden, so that might be another explanation.