A white mushroom grew overnight out of nowhere, is it a risk to the plant?

It might be because the cold doesn’t allow the substrate moisture to dry properly. Adjust temperature and ventilation.

First of all, welcome @Kalli and @Enzoo.
With cheap soil, something like this can happen with fungi. Usually, these are fungi that decompose wood or other substances in the soil.

A picture would help identify the mushroom which would narrow down the cause. In living soil mushrooms are common due to mycelium and other factors. Most commonly what’s called flower pot fungus which is a sign of healthy soil and nothing to worry about. I also use bricks of straw used to grow mushrooms as mulch which helps cause shrooms to grow. In a synthetic grow system shrooms are much less common but still can indicate that the soil food web is healthy. It can also indicate over watering in any grow system.

One of these!
IMG_2985|576x384

It’s in the forest, isn’t it?

I don’t think it’s flower pot fungus they have more of a yellowish color to them. It doesn’t look like anything I’ve experienced. Wish I could be of more help. I will say it’s a sign you have a healthy fungal network which will help with nutrient uptake.

Hey friend, my grow is indoor, I got this photo from Google as a reference, because I took the mushroom out of the pot and forgot to take a picture.

Thanks for the explanation, friend.

No problem at all. There’s life in the pot. Just control watering. Try using perlite next time. It provides more oxygen to the soil and is easier for the roots. A ratio of 1 part perlite to 4 parts soil.