On closer inspection it looks like threads of white compacted together.
I suspect it is fungal mycelium growing on a layer of organic matter.
https://en.wikipedia.org/wiki/Mycelium
But there is a chance it is a natural high calcium chalky substance, or layer of spilled or dumped cement spoils or paint from a building project.
It has appeared over night on a old tree stump in my back garden.
From the photo it appears to be a fruiting body (spore dispersal organ, similar to a mushroom) of a wood decay fungus.
It is most likely infects only dead wood as part of the decay process, so not of concern for your living trees and shrubs.
We feel like we are suffering a plague with our discovering and removing stinkhorns both in the white golfball egg stage and in the putrid orange stage, over a period of 9 months at our residence in Coastal Alabama. With no exaggeration, since first discovering in June 2017, we have removed easily over 100 from our back yard plant beds AND our front yard plant beds. These beds were filled with new mulch we purchased and installed after the removal of several trees and stumps being ground and removed from our yard. After researching on line, as soon as we discover one, we dig it up along with any attached soil and hairlike roots, place in a large zip lock bag and put in our garbage to be picked up and disposed of. For the odor sake, we spray the open hole with household mold wash and leave it open to the air. Our daily routine has become, especially following a rain, is that we walk around and thoroughly inspect the beds in search of more. Help!! We feel we have been invaded - any insight, treatment options or additional information would be great! Thank you, The Smiths rysmithster@gmail.com
This pest will have to run its course, naturally. They will eventually die out, but there is nothing that you can do to kill them off. There are only preventative measures that can reduce the number until they resolve themselves. This article will help: https://www.gardeningknowhow.com/ornamental/fungus-lichen/removing-stinkhorn-fungi.htm
Can a infrared light be used to keep mushrooms warm & in the dark or will the light interfere with the needed darkness??
I would avoid the infrared light.
This article will help you.
https://www.gardeningknowhow.com/edible/vegetables/mushrooms/grow-mushrooms.htm
Wondering what kind of mushroom this is and why is it growing in flower pots?
This article will help you.
https://www.gardeningknowhow.com/houseplants/hpgen/getting-rid-of-mushrooms-growing-in-houseplant-soil.htm
Came out after days of rains (about 1 inch) mid 40 degree farenheit, to mysterious chunky piles of gel in my pots! Thought it was an animal, have deer and rabbit evidence, but some of the pots are unreachable, no damage to plants. Not dissolving after two days. Not related to new plantings. Fungus? Chemicals from the air? Birds? Thanks, gardeners!
It looks like you have Slime Mold.
This article will help you.
https://www.gardeningknowhow.com/ornamental/fungus-lichen/what-is-slime-mold-slime.htm
Can anyone help me identify what is growing in my garden and how to stop it? See attached picture. It disintegrates into powder when I try to dig it up.
You appear to have Slime Mold growing in your flower bed.
This article will help you. https://www.gardeningknowhow.com/ornamental/fungus-lichen/what-is-slime-mold-slime.htm