Mushrooms in cat grass I am growing

I thought that it might be a nice treat if I were to grow a little garden of cat grass for my cat to munch on throughout the day.

I have been growing the grass in a larger rectangular pot (the kind meant to hang off of balconies) in my cat's kitten room (yes he has his own room )

The first few months were great, he loved eating the grass before his breakfast, and enjoyed pawing around in the dirt.

Recently the supply of grass got dry, and much of it was eaten or died away. I replanted a number of seeds, added more soil, and watered the pot.

I must have watered it too much because all of a sudden mushrooms are popping up all over the place! They are a white mushroom, with a very light power covering them that looks to be a light greeny-yellow color.

I immediately plucked the mushrooms out, but several more came back when I woke up this morning. I have now locked the pot away from my cat until I can figure out if the mushrooms are harmful.

Has anyone else come across this problem? Should I be concerned about the mushrooms being poisonous? How can I prevent this from happening in the future?

****FYI, I was using Miracle Grow moisture control potting mix. It was recommended when I researched growing cat grass, but I have since learned that it is very common to have mushrooms grow in this soil.

Yeah, I was going to say it might be the potting soil you used. I would recommend getting a different soil and starting over.

They are most probably not poisonous. To you, that is - you still wouldn't want any cat to eat them...   if you're cautious, trow it out and try again with a different kind of soil. Best something organic, untreated because the cat might rip some grass out, dig around and ingest the soil.

The problem here is fertilizer in the soil mix - the Rogue, who eats everything he shouldn't, snarfed down some miracle grow soil once and ended up at the vet with dangerously high phosphorus in the blood

My niece is a cat lover and in the plant/nursery industry. She's not real particular about what SHE eats. But when I first adopted Ritz and asked her how to grow cat grass, she emphasized that the soil should be organic and be very careful about the fertilizer you use (if any).
Also some cats throw up cat grass, some even eat for just that purpose, to get something out of their system.
Just a word of caution.....

For the price of a pot of cat grass, I would just throw it out.