Answer: Although you didn't describe the growth as mushroom-like, I suspect what you are seeing is a fungal growth of some kind. It is quite common, especially where organic matter is decomposing and the area is moist. Fungus like this grows from decaying bark, dead roots and even compost which is used when installing plants. It's harmless to you and to your plants (it's a natural growth due to decomposition of organic matter). Pick it up with a shovel or with your hands and toss it in the garbage.
Best wishes with your new landscape!
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