Certified GKH Gardening Expert
Houseplant mushrooms are usually of the species described in this article:
This species is not considered to be very toxic, but reports vary, and they certainly shouldn't be eaten by people or animals. And if any other species of mushroom is growing in your houseplant soil, it could be a very toxic variety. For example, this one:
For those reasons, if your cat has eaten one, it is best to get him checked out just in case.