It has grown from a small pearl size ball to a kind of toadstool-like shape in 3 days. What is it? Should I be concerned?

I think it may be Leucocoprinus birnbaumii. For more info on it:
http://www.mushroomexpert.com/...

They are known for cropping up in potted plants.

I forgot to say to ATP!

Welcome indeed! I get those in my succulent pots as well .. does no harm but you can dig them out if you wish. They seem to pop up in potting mix that has 'forest products' like ground up or shredded hardwood bark and wood.

Hi Vicsav, and a big to ATP.
Great photo of mushroom and sempervivum.
I also have the mushrooms pop up occasionally amongst the sempervivum. I agree that it does them no harm. I usually remove them. Most of the compost we get for gardening contains forest products. I have since found a wonderful compost that contains very little forest compost, it is a sandy loam mix that the semps love. Even the vegetable plants seem to love it.
Hope you will come visit us in the Sempervivum Forum and share your sempervivum photos. http://garden.org/forums/view/...

valleylynn said:. I have since found a wonderful compost that contains very little forest compost, it is a sandy loam mix that the semps love. Even the vegetable plants seem to love it.
Hope you will come visit us in the Sempervivum Forum and share your sempervivum photos. http://garden.org/forums/view/...

Welcome to ATP and to visit the Semp forums.

@valleylynn what is the product to which you refer?

Marilyn, do you mean the sandy loam?
Here is a great thread that discusses just what sandy loam is. The thread "define Loamy" in All Things Gardening forum

Cutie mushroom! I get those too when I have kept the soil a tad more moist. They do not harm the plants. When I see those, it is a good reminder for me to stop watering for awhile..soil too wet.

Lynn I thought you had a product you had found that was helpful in building the sandy loam.
I know you sometime talk about Happy Frog soil.

I get the sandy loam compost from our local farm store. I do use Happy Frog for some of the container plants, but not the semps.
This is the one I use. Love it, and so do the plants.

Thanks to all who replied to my question about the yellow alien-looking mushrooms invading my hens and chicks. It has been rather humid this past week and there are wood fragments in the potting mix. Posting another pic showing the growth in just 24 hrs!

Wow, that is pretty fast growing. What a fascinating thing to watch. I think I would leave it and watch it to the end of it's growing cycle. Very interesting.

I love the Birds Nest fungus, Cyathus olla that appear in my pots from time to time .. it's so unusual and pretty!