What is this white fluff growing in my basement?

Question

I live in the countryside of Western Europe by the beach with a temperate climate. After a period of unusual humidity at 84%, I noted this white fluff in the basement floor:

The first picture is about 40 cm and the second 10 cm.

The basement is usually closed and keeps humidity. Temperatures average 18-20ºC, and humidity is now 77%. I assume it's a fungus since the floor was clear before and it seems to collapse when I rub my foot on it.

What is this species, and how can I prevent it?

Update: Following @anongoodnurse's suggestions, it's definitely a salt. I scraped the stuff, poured water, and it shriveled instantly. Here are photos before and after:

Here is a close-up of another piece, where the shards look like a crystal:

It did not smell indeed, and I did not taste it. I opened the basement door to ventilate the area, and I understand that it would accelerate the evaporation of the salty water.

I don't know if this growth has happened before, e.g. because of a leak in a pipe, or is a one-off occurrence, e.g. because of the unusual humidity in the last few weeks. I will check if the basement has any pipes.

Answer 1

Might as well make this an answer.

...it seems to collapse when I rub my foot on it.

As @tyersome stated, it appears to be salt, not mold. Salt crystals would definitely collapse easily underfoot, and should be mostly quite dry, whereas mold would be... maybe squishy (?) but likely unpleasant in some way. Mold also smells, which you haven't mentioned, whereas salt is odorless.

To test, scrape a bit up (salt crystals should feel a bit sandy, not mossy) and see what a drop or two of water does. Mold doesn't dissolve in water; it floats. If water seems to dissolve some of it, add more. If it all dissolves, it's probably a salt, and since you live near the beach, it might be mostly salt, i.e. NaCl, with trace minerals thrown in.

If it dissolves and you're brave, you can taste a tiny bit then spit and rinse. While salt isn't toxic, I can't vouch for what was in the grout.

My guess is that there may be a low spot or two in your basement where salt water is entering through the porous grout; as it evaporates, it leaves salt crystals behind. More water = saltier solution = bigger crystals as the water evaporates.

If you have a magnifying glass, you should see crystals in the undisturbed deposit. If instead it looks soft and hairy, it is mold. I can't think of a white mold off the top of my head (well, Candida albicans, but that smells yeasty and is definitely not crumbly). Mold/yeast needs a substrate (food) to grow on, and wet grout isn't very nutritious. Mildew, sure, but mot big fluffy mold.

_{I don't like this reference because it's not scientific, but it explains efflorescence well.)}